Device and Device Synchronization Method in Device-to-Device (D2D) Communication

ABSTRACT

A device and a device synchronization method in D 2 D communication, so as to implement synchronization between devices in D 2 D communication is provided. The device includes: a receiving unit, configured to receive first D 2 D synchronization information sent by a first device in D 2 D communication, where the first D 2 D synchronization information includes a synchronization source identifier and a forwarding hop count. The device also includes a processing unit, configured to synchronize with a synchronization source corresponding to the synchronization source identifier in the first D 2 D synchronization information, and update the forwarding hop count in the first D 2 D synchronization information when determining that the forwarding hop count in the first D 2 D synchronization information is less than a set threshold, so as to obtain second D 2 D synchronization information. The device also includes a sending unit, configured to send the second D 2 D synchronization information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/CN2014/071373, filed on Jan. 24, 2014, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to the field of communications technologies, and in particular, to a device and a device synchronization method in device-to-device (D₂D) communication.

BACKGROUND

In a device-to-device (D₂D) communication manner, devices can communicate with each other by using a particular mechanism without transit by a base station; therefore, communication can be implemented in a place where network facilities are inadequate or no network is available. However, during D₂D communication, each device has a right and an opportunity to initiate communication, but stability of time and frequency of user equipment (UE) is poorer than that of a base station, and different UEs apply different time and frequency references. If each UE randomly transmits a synchronization signal, receiver UE will be unable to determine a synchronization reference, and as a result, communication cannot be established.

In a Long Term Evolution (LTE) system, a synchronization mechanism based on a two-step method is provided. The synchronization mechanism involves one primary synchronization signal (PSS) and one secondary synchronization signal (SSS). A cell identifier is allocated to a base station, and the PSS and the SSS are used to indicate the cell identifier. UE selects a serving cell after power-on and synchronizes to the selected serving cell by using the PSS and the SSS. That is, UEs that belong to different cells synchronize to different base stations. A synchronization method is: UE receives, by using a receiver, a PSS and an SSS that are sent by a base station and tracks the PSS and the SSS, and aligns time and frequency with that of a base station that provides a service for the UE. Base stations synchronize time and frequency to a unified synchronization source by using a global positioning system (GPS), or the like.

In an LTE downlink communication mechanism, in a cell, only one transmitter transmits a synchronization signal, and all UEs synchronize to the cell. However, in a D₂D scenario, each UE may send a synchronization signal, and an LTE two-step synchronization mechanism does not suit such a distributed network structure of D₂D. It can be seen that how to implement synchronization between devices in D₂D communication is an issue to be resolved.

SUMMARY

Embodiments provide a device and a device synchronization method in D₂D communication, so as to implement synchronization between devices in D₂D communication.

Specific technical solutions provided in the embodiments are as follows.

According to a first aspect, a device in device-to-device D₂D communication is provided. The device includes a receiving unit, configured to receive first D₂D synchronization information sent by a first device in D₂D communication, where the first D₂D synchronization information includes a synchronization source identifier and a forwarding hop count. The device also includes a processing unit, configured to synchronize with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information; and update the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information. The device also includes a sending unit, configured to send the second D₂D synchronization information.

With reference to the first aspect, in a first possible implementation manner, the first D₂D synchronization information received by the receiving unit is carried in a D₂D synchronization signal sent by the first device; or the first D₂D synchronization information received by the receiving unit includes first-part D₂D synchronization information and second-part D₂D synchronization information, where the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel PD₂DSCH.

With reference to the first possible implementation manner of the first aspect, in a second possible implementation manner, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH are different.

With reference to the first possible implementation manner of the first aspect, in a third possible implementation manner, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

With reference to the third possible implementation manner of the first aspect, in a fourth possible implementation manner, the processing unit is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; and after the PD₂DSS in the D₂D synchronization signal received by the receiving unit is replaced with the determined PD₂DSS, use a D₂D synchronization signal with the replaced PD₂DSS as the second D₂D synchronization information.

With reference to the third possible implementation manner of the first aspect, in a fifth possible implementation manner, the processing unit is specifically configured to: determine a synchronization source identifier according to indication information, received by the receiving unit based on the PD₂DSCH, of the synchronization source identifier; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit. The processing unit is also configured to, when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; and after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, use a D₂D synchronization signal obtained after PD₂DSS replacement as first-part D₂D synchronization information of the second D₂D synchronization information, and use the indication information of the synchronization source identifier as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the third possible implementation manner of the first aspect, in a sixth possible implementation manner, the processing unit is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to determine a forwarding hop count according to indication information, received by the receiving unit based on the PD₂DSCH, of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and use the D₂D synchronization signal that carries the SD₂DSS as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the third possible implementation manner of the first aspect, in a seventh possible implementation manner, the processing unit is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, a synchronization source identifier and a forwarding hop count that are corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to, when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the synchronization source identifier and the current forwarding hop count; replace the SD₂DSS in the D₂D synchronization signal with the determined SD₂DSS; and use a D₂D synchronization signal obtained after SD₂DSS replacement as the second D₂D synchronization information.

With reference to the seventh possible implementation manner of the first aspect, in an eighth possible implementation manner, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

With reference to the third possible implementation manner of the first aspect, in a ninth possible implementation manner, the processing unit is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a synchronization source identifier that is corresponding to a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is further configured to determine a forwarding hop count according to indication information, received by the receiving unit based on the PD₂DSCH, of the forwarding hop count; and when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count. The processing unit is further configured to use the D₂D synchronization signal as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the ninth possible implementation manner of the first aspect, in a tenth possible implementation manner, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

With reference to the third possible implementation manner of the first aspect, in an eleventh possible implementation manner, the processing unit is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a synchronization source identifier that is corresponding to a PD₂DSS-SD₂DSS group to which a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit belongs; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine, according to a preset correspondence between an SD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the SD₂DSS in the combination of the PD₂DSS and the SD₂DSS that are carried in the received D₂D synchronization; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; acquire, according to the preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the current forwarding hop count; determine a combination of a PD₂DSS and an SD₂DSS that is corresponding to the acquired SD₂DSS and that is included in the PD₂DSS-SD₂DSS group to which the combination of the PD₂DSS and the SD₂DSS carried in the D₂D synchronization signal belongs; replace the combination of the PD₂DSS and the SD₂DSS in the D₂D synchronization signal with the determined combination of the PD₂DSS and the SD₂DSS; and use a D₂D synchronization signal obtained after replacement of the combination of the PD₂DSS and the SD₂DSS as the second D₂D synchronization information.

With reference to the eleventh possible implementation manner of the first aspect, in a twelfth possible implementation manner, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

With reference to the twelfth possible implementation manner of the first aspect, in a thirteenth possible implementation manner, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

With reference to any one of the first aspect to the thirteenth possible implementation manner of the first aspect, in a fourteenth possible implementation manner, the processing unit is further configured to: when determining that the receiving unit receives more than one piece of first D₂D synchronization information, separately acquire forwarding hop counts carried in the received pieces of first D₂D synchronization information; determine a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronize with a synchronization source corresponding to a synchronization source identifier in the first D₂D synchronization information corresponding to the minimum value.

With reference to the fourteenth possible implementation manner of the first aspect, in a fifteenth possible implementation manner, the processing unit is specifically configured to: when the minimum value is corresponding to more than one piece of first D₂D synchronization information, determine signal strengths of the pieces of first D₂D synchronization information corresponding to the minimum value; and perform synchronization according to the first D₂D synchronization information corresponding to a greatest value of the signal strengths.

With reference to any one of the first aspect to the fourteenth possible implementation manner of the first aspect, in a sixteenth possible implementation manner, the first D₂D synchronization information further carries a synchronization source type. The processing unit is specifically configured to: when determining that the receiving unit receives more than one piece of first D₂D synchronization information, determine, according to synchronization source types carried in the pieces of first D₂D synchronization information, the first D₂D synchronization information in which a synchronization source is a network; and perform synchronization according to the determined first D₂D synchronization information.

With reference to the sixteenth possible implementation manner of the first aspect, in a seventeenth possible implementation manner, the processing unit is specifically configured to: after determining the first D₂D synchronization information in which the synchronization source is a network, acquire a forwarding hop count carried in the determined first D₂D synchronization information in which the synchronization source is a network; and if determining that the forwarding hop count is less than the set threshold, perform synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network; or if determining that the forwarding hop count is not less than the set threshold, acquire forwarding hop counts carried in the other pieces of first D₂D synchronization information; determine a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronize with a synchronization source indicated by a synchronization source identifier in the first D₂D synchronization information corresponding to the minimum value.

With reference to the sixteenth possible implementation manner of the first aspect, in an eighteenth possible implementation manner, the processing unit is specifically configured to: update a greatest value of the forwarding hop counts, where a greatest value of the forwarding hop counts that is obtained after the update is greater than the greatest value of the forwarding hop counts that exists before the update; and when determining that the forwarding hop count carried in the first D₂D synchronization information is less than the greatest value of the forwarding hop counts that is obtained after the update, update the forwarding hop count in the first D₂D synchronization information, to obtain the second D₂D synchronization information, where the sending unit sends the second D₂D synchronization information, and the sending unit sends the greatest value of the forwarding hop counts that is obtained after the update.

With reference to any one of the first possible implementation manner to the fourteenth possible implementation manner of the first aspect, in a nineteenth possible implementation manner, the sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, transmit a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

With reference to any one of the first possible implementation manner to the fourteenth possible implementation manner of the first aspect, in a twentieth possible implementation manner, the sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, send a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

With reference to any one of the first possible implementation manner to the fourteenth possible implementation manner of the first aspect, in a twenty-first possible implementation manner, the sending unit is specifically configured to: after determining that a D₂D synchronization signal needs to be sent, send only the D₂D synchronization signal within a set quantity of transmission cycles; and the receiving unit does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

According to a second aspect, a device in device-to-device D₂D communication is provided. The device includes a message processing unit, configured to determine a synchronization source identifier and a forwarding hop count; and encapsulate the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information. The device also includes a sending unit, configured to send the first D₂D synchronization information processed by the message processing unit.

With reference to the second aspect, in a first possible implementation manner, the first D₂D synchronization information further includes a synchronization source type. The device further includes a detection unit and a receiving unit, where the detection unit is configured to detect a downlink synchronization signal sent by a network. The message processing unit is further configured to: when determining that the detection unit has not detected the downlink synchronization signal, instruct the sending unit to send the first D₂D synchronization information; or after determining that the detection unit has detected the downlink synchronization signal, determine that the receiving unit has received third D₂D synchronization information sent by another device; determine, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device; and instruct the sending unit to send the first D₂D synchronization information.

With reference to the first possible implementation manner of the second aspect, in a second possible implementation manner, a time-frequency occupied by the first D₂D synchronization information that is sent by the sending unit is different from a time-frequency resource occupied by the downlink synchronization signal that is sent by the network and that is received by the receiving unit.

With reference to the first possible implementation manner of the second aspect, in a third possible implementation manner, the sending unit is specifically configured to: send a D₂D synchronization signal, where the D₂D synchronization signal carries the first D₂D synchronization information; or send a D₂D synchronization signal, where the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information; and send second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel PD₂DSCH, where the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the first D₂D synchronization information.

With reference to the third possible implementation manner of the second aspect, in a fourth possible implementation manner, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH are different.

With reference to the third possible implementation manner of the second aspect, in a fifth possible implementation manner, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

With reference to the fifth possible implementation manner of the second aspect, in a sixth possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; add the acquired PD₂DSS and SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal that carries the PD₂DSS and the SD₂DSS as the first D₂D synchronization information.

With reference to the fifth possible implementation manner of the second aspect, in a seventh possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; add the acquired PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal that carries the PD₂DSS as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the determined synchronization source identifier as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

With reference to the fifth possible implementation manner of the second aspect, in an eighth possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; and add the acquired SD₂DSS and a preset PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information, where PD₂DSSes preset for all synchronization sources are the same in D₂D communication.

With reference to the fifth possible implementation manner of the second aspect, in a ninth possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count; and add the SD₂DSS to the D₂D synchronization signal, and use the D₂D synchronization signal as the first D₂D synchronization information.

With reference to the ninth possible implementation manner of the second aspect, in a tenth possible implementation manner, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

With reference to the fifth possible implementation manner of the second aspect, in an eleventh possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a combination of a PD₂DSS and an SD₂DSS that corresponds to the determined synchronization source identifier; and add the acquired combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

With reference to the eleventh possible implementation manner of the second aspect, in a twelfth possible implementation manner, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

With reference to the fifth possible implementation manner of the second aspect, in a thirteenth possible implementation manner, the message processing unit is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier, where each PD₂DSS-SD₂DSS group includes multiple combinations of a PD₂DSS and an SD₂DSS; acquire, according to a preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined forwarding hop count; and determine a combination of a PD₂DSS and an SD₂DSS that includes the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group; and add the determined combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal as the first D₂D synchronization information.

With reference to the thirteenth possible implementation manner of the second aspect, in a fourteenth possible implementation manner, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

With reference to the fourteenth possible implementation manner of the second aspect, in a fifteenth possible implementation manner, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

With reference to any one of the third possible implementation manner to the fifteenth possible implementation manner of the second aspect, in a sixteenth possible implementation manner, the message processing unit is further configured to: convert, according to a set cycle, the synchronization source identifier carried in the first D₂D synchronization information.

With reference to the sixteenth possible implementation manner of the second aspect, in a seventeenth possible implementation manner, the message processing unit is further configured to: convert, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal; or convert, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal; or convert, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal, and change, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal.

With reference to the seventeenth possible implementation manner of the second aspect, in an eighteenth possible implementation manner, the message processing unit is further configured to: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI+Nframe(m)Ns), where

N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, Nframe(m) represents a number of a radio frame on which the device is located when the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal is changed for the m^(th) time, and N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources.

With reference to the seventeenth possible implementation manner of the second aspect, in a nineteenth possible implementation manner, the message processing unit is further configured to: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI×(N_(ID) ⁽²⁾(m-1)+a),Ns), where

when m is equal to 0, N_(ID) ⁽²⁾(0)=mod(IMSI+Nframe(0),Ns), N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources, a represents a non-zero integer, and Nframe (0) represents a number of a radio frame on which the device is located when the D₂D synchronization signal is sent for the first time.

With reference to any one of the third possible implementation manner to the fifteenth possible implementation manner of the second aspect, in a twentieth possible implementation manner, the sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, transmit a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

With reference to any one of the third possible implementation manner to the fifteenth possible implementation manner of the second aspect, in a twenty-first possible implementation manner, the sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, send a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

With reference to any one of the third possible implementation manner to the fifteenth possible implementation manner of the second aspect, in a twenty-second possible implementation manner, the sending unit is specifically configured to: after determining that a D₂D synchronization signal needs to be sent, send only the D₂D synchronization signal within a set quantity of transmission cycles; and the receiving unit does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

According to a third aspect, a method for sending synchronization information in device-to-device D₂D communication is provided. The method includes determining, by a first device in D₂D communication, a synchronization source identifier and a forwarding hop count; and encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information. The method also includes sending the first D₂D synchronization information, where a second device in D₂D communication receives the first D₂D synchronization information; synchronizes with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information; and updates the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold and sends second D₂D synchronization information obtained after update of the forwarding hop count.

With reference to the third aspect, in a first possible implementation manner, the first D₂D synchronization information further includes a synchronization source type. Before the sending the first D₂D synchronization information, the method includes: detecting, by the first device, a downlink synchronization signal sent by a network; and determining that the downlink synchronization signal is not detected; or detecting, by the first device, a downlink synchronization signal sent by a network; after determining that the downlink synchronization signal is detected, receiving third D₂D synchronization information sent by another device; and determining, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device.

With reference to the first possible implementation manner of the third aspect, in a second possible implementation manner, a time-frequency occupied by the first D₂D synchronization information is different from a time-frequency resource occupied by the downlink synchronization signal sent by the network.

With reference to the third aspect, in a third possible implementation manner, the sending the first D₂D synchronization information includes: sending a D₂D synchronization signal, where the D₂D synchronization signal carries the first D₂D synchronization information; or sending a D₂D synchronization signal, where the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information; and sending second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel PD₂DSCH, where the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the first D₂D synchronization information.

With reference to the third possible implementation manner of the third aspect, in a fourth possible implementation manner, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH are different.

With reference to the fourth possible implementation manner of the third aspect, in a fifth possible implementation manner, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

With reference to the fifth possible implementation manner of the third aspect, in a sixth possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; acquiring, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; adding the acquired PD₂DSS and SD₂DSS to the D₂D synchronization signal; and using the D₂D synchronization signal as the first D₂D synchronization information.

With reference to the fifth possible implementation manner of the third aspect, in a seventh possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; adding the acquired PD₂DSS to the D₂D synchronization signal; using the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and using indication information of the determined synchronization source identifier as the second-part D₂D synchronization information that is sent based on the PD₂DSCH and that is of the first D₂D synchronization information.

With reference to the fifth possible implementation manner of the third aspect, in an eighth possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; and adding the acquired SD₂DSS and a preset PD₂DSS to the D₂D synchronization signal; using the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and using indication information of the determined forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information, where PD₂DSSes preset for all synchronization sources are the same in D₂D communication.

With reference to the fifth possible implementation manner of the third aspect, in a ninth possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count; and adding the SD₂DSS to the D₂D synchronization signal, and using the D₂D synchronization signal as the first D₂D synchronization information.

With reference to the ninth possible implementation manner of the third aspect, in a tenth possible implementation manner, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

With reference to the fifth possible implementation manner of the third aspect, in an eleventh possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a combination of a PD₂DSS and an SD₂DSS that corresponds to the determined synchronization source identifier; and adding the acquired combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; using the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and using indication information of the determined forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

With reference to the eleventh possible implementation manner of the third aspect, in a twelfth possible implementation manner, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

With reference to the fifth possible implementation manner of the third aspect, in a thirteenth possible implementation manner, the encapsulating the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information includes: acquiring, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier, where each PD₂DSS-SD₂DSS group includes multiple combinations of a PD₂DSS and an SD₂DSS; acquiring, according to a preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined forwarding hop count; and determining a combination of a PD₂DSS and an SD₂DSS that includes the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group; and adding the determined combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; and using the D₂D synchronization signal as the first D₂D synchronization information.

With reference to the thirteenth possible implementation manner of the third aspect, in a fourteenth possible implementation manner, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

With reference to the fourteenth possible implementation manner of the third aspect, in a fifteenth possible implementation manner, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

With reference to any one of the fifth possible implementation manner to the fifteenth possible implementation manner of the third aspect, in a sixteenth possible implementation manner, the determining a synchronization source identifier includes: converting, according to a set cycle, the synchronization source identifier carried in the first D₂D synchronization information.

With reference to the sixteenth possible implementation manner of the third aspect, in a seventeenth possible implementation manner, the converting, according to a set cycle, the synchronization source identifier carried in the first D₂D synchronization information includes: converting, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal, and changing, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal.

With reference to the seventeenth possible implementation manner of the third aspect, in an eighteenth possible implementation manner, the converting, according to the set cycle, the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal includes: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI+Nframe(m),Ns), where

N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, Nframe(m) represents a number of a radio frame on which the device is located when the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal is changed for the m^(th) time, and N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources.

With reference to the seventeenth possible implementation manner of the third aspect, in a nineteenth possible implementation manner, the converting, according to the set cycle, the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal includes: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI×(N_(ID) ⁽²⁾(m-1)+a),Ns), where when m is equal to 0, N_(ID) ⁽²⁾(0)=mod(IMSI+Nframe(0),Ns, N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources, a represents a non-zero integer, and Nframe(0) represents a number of a radio frame on which the first device is located when the D₂D synchronization signal is sent for the first time.

With reference to any one of the fifth possible implementation manner to the fifteenth possible implementation manner of the third aspect, in a twentieth possible implementation manner, the sending the first D₂D synchronization information includes: when receiving a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, transmitting a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

With reference to any one of the fifth possible implementation manner to the fifteenth possible implementation manner of the third aspect, in a twenty-first possible implementation manner, the sending the first D₂D synchronization information includes: when receiving a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, sending a D₂D synchronization signal by using another resource that is used for a D₂D synchronization signal and that is included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

With reference to any one of the fifth possible implementation manner to the fifteenth possible implementation manner of the third aspect, in a twenty-second possible implementation manner, the sending the first D₂D synchronization information includes: after determining that a D₂D synchronization signal needs to be sent, sending, by the first device, only the D₂D synchronization signal within a set quantity of transmission cycles, and skipping receiving any D₂D synchronization signal within the set quantity of transmission cycles.

According to a fourth aspect, a device synchronization method in device-to-device D₂D communication is provided. The method includes receiving, by a second device in D₂D communication, first D₂D synchronization information sent by a first device, where the first D₂D synchronization information includes a synchronization source identifier and a forwarding hop count. The method also includes synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information, and updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information. The method also includes sending the second D₂D synchronization information.

With reference to the fourth aspect, in a first possible implementation manner, the first D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device; or the first D₂D synchronization information includes first-part D₂D synchronization information and second-part D₂D synchronization information, where the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel PD₂DSCH.

With reference to the first possible implementation manner of the fourth aspect, in a second possible implementation manner, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH are different.

With reference to the first possible implementation manner of the fourth aspect, in a third possible implementation manner, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

With reference to the third possible implementation manner of the fourth aspect, in a fourth possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: determining, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS and that is carried in the received D₂D synchronization signal; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier; and the updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: determining, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS and that is carried in the received D₂D synchronization signal; and when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determining, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; and after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, using a D₂D synchronization signal with the replaced PD₂DSS as the second D₂D synchronization information.

With reference to the third possible implementation manner of the fourth aspect, in a fifth possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: receiving, based on the PD₂DSCH, indication information of the synchronization source identifier; determining the synchronization source identifier according to the indication information of the synchronization source identifier; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier. The updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: determining, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS and that is carried in the received D₂D synchronization signal; and when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determining, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; replacing the PD₂DSS in the received D₂D synchronization signal with the determined PD₂DSS; and using a D₂D synchronization signal obtained after PD₂DSS replacement as first-part D₂D synchronization information of the second D₂D synchronization information, and using the indication information of the synchronization source identifier as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the third possible implementation manner of the fourth aspect, in a sixth possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: determining, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS and that is carried in the received D₂D synchronization signal; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier. The updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: receiving, based on the PD₂DSCH, indication information of the forwarding hop count; determining the forwarding hop count according to the indication information of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and using the synchronization signal that carries the SD₂DSS as first-part D₂D synchronization information of the second D₂D synchronization information, and using indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the third possible implementation manner of the fourth aspect, in a seventh possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information, and updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: determining, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, a synchronization source identifier and a forwarding hop count that are corresponding to the SD₂DSS carried in the received D₂D synchronization signal; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier; and when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determining, according to the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the synchronization source identifier and the current forwarding hop count; replacing the SD₂DSS in the D₂D synchronization signal with the determined SD₂DSS; and using a D₂D synchronization signal obtained after SD₂DSS replacement as the second D₂D synchronization information.

With reference to the seventh possible implementation manner of the fourth aspect, in an eighth possible implementation manner, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

With reference to the third possible implementation manner of the fourth aspect, in a ninth possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: determining, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a synchronization source identifier that is corresponding to a combination of the PD₂DSS and the SD₂DSS and the SD2SS carried in the received D₂D synchronization signal; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier; and the updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: receiving, based on the PD₂DSCH, indication information of the forwarding hop count; determining the forwarding hop count according to the indication information of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and using the D₂D synchronization signal as first-part D₂D synchronization information of the second D₂D synchronization information, and using indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

With reference to the ninth possible implementation manner of the fourth aspect, in a tenth possible implementation manner, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

With reference to the third possible implementation manner of the fourth aspect, in an eleventh possible implementation manner, the synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: determining, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a synchronization source identifier that is corresponding to a PD₂DSS-SD₂DSS group to which a combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs; and synchronizing with a synchronization source corresponding to the determined synchronization source identifier; and the updating the forwarding hop count in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information includes: determining, according to a preset correspondence between an SD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the SD₂DSS in the combination of the PD₂DSS and the SD₂DSS that are carried in the received D₂D synchronization signal; when the determined forwarding hop count is less than the set threshold, using a sum of the determined forwarding hop count and a set value as a current forwarding hop count; acquiring, according to the preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the current forwarding hop count; determining a combination of a PD₂DSS and an SD₂DSS that is corresponding to the acquired SD₂DSS and that is included in the PD₂DSS-SD₂DSS group to which the combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs; replacing the combination of the PD₂DSS and the SD₂DSS in the D₂D synchronization signal with the determined combination of the PD₂DSS and the SD₂DSS; and using a D₂D synchronization signal obtained after replacement of the combination of the PD₂DSS and the SD₂DSS as the second D₂D synchronization information.

With reference to the eleventh possible implementation manner of the fourth aspect, in a twelfth possible implementation manner, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

With reference to the twelfth possible implementation manner of the fourth aspect, in a thirteenth possible implementation manner, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

With reference to any one of the fourth aspect to the thirteenth possible implementation manner of the fourth aspect, in a fourteenth possible implementation manner, the receiving first D₂D synchronization information, and synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: receiving the first D₂D synchronization information; when determining that more than one piece of first D₂D synchronization information is received, separately acquiring forwarding hop counts carried in the received pieces of first D₂D synchronization information; determining a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronizing with a synchronization source corresponding to a synchronization source identifier in first D₂D synchronization information corresponding to the minimum value.

With reference to the fourteenth possible implementation manner of the fourth aspect, in a fifteenth possible implementation manner, the performing synchronization according to first D₂D synchronization information corresponding to the minimum value includes: if the minimum value is corresponding to more than one piece of first D₂D synchronization information, determining signal strengths of the pieces of first D₂D synchronization information corresponding to the minimum value; and performing synchronization according to the first D₂D synchronization information corresponding to a greatest value of the signal strengths.

With reference to any one of the fourth aspect to the thirteenth possible implementation manner of the fourth aspect, in a sixteenth possible implementation manner, the receiving first D₂D synchronization information, and synchronizing with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information includes: receiving the first D₂D synchronization information; when determining that more than one piece of first D₂D synchronization information is received, determining, according to synchronization source types carried in the first D₂D synchronization signals, the first D₂D synchronization information in which a synchronization source is a network; and performing synchronization according to the determined first D₂D synchronization information.

With reference to the sixteenth possible implementation manner of the fourth aspect, in a seventeenth possible implementation manner, after the first D₂D synchronization information in which the synchronization source is a network is determined, the method further includes: acquiring a forwarding hop count carried in the determined first D₂D synchronization information in which the synchronization source is a network; and if determining that the forwarding hop count is less than the set threshold, performing synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network; or if determining that the forwarding hop count is not less than the set threshold, acquiring forwarding hop counts carried in the other pieces of first D₂D synchronization information; determining a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronizing with a synchronization source indicated by a synchronization source identifier in the first D₂D synchronization information corresponding to the minimum value.

With reference to the sixteenth possible implementation manner of the fourth aspect, in an eighteenth possible implementation manner, the determining the first D₂D synchronization information in which a synchronization source is a network, and performing synchronization according to the determined first D₂D synchronization information further includes: updating a greatest value of the forwarding hop counts, where a greatest value of the forwarding hop counts that is obtained after the update is greater than the greatest value of the forwarding hop counts that exists before the update; and when determining that the forwarding hop count in the first D₂D synchronization information is less than the greatest value of the forwarding hop counts that is obtained after the update, updating the forwarding hop count in the first D₂D synchronization information, to obtain the second D₂D synchronization information; sending the second D₂D synchronization information, and sending the greatest value of the forwarding hop counts that is obtained after the update.

With reference to any one of the first possible implementation manner of the fourth aspect to the thirteenth possible implementation manner of the fourth aspect, in a nineteenth possible implementation manner, the method further includes: when a D₂D synchronization signal is received by using any one of radio frames used for sending a D₂D synchronization signal, transmitting a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

With reference to any one of the first possible implementation manner of the fourth aspect to the thirteenth possible implementation manner of the fourth aspect, in a twentieth possible implementation manner, the method further includes: when a D₂D synchronization signal is received by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, sending a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

With reference to any one of the first possible implementation manner of the fourth aspect to the thirteenth possible implementation manner of the fourth aspect, in a twenty-first possible implementation manner, the method further includes: after the second device determines that a D₂D synchronization signal needs to be sent, sending only the D₂D synchronization signal within a set quantity of transmission cycles; and skipping receiving any D₂D synchronization signal within the set quantity of transmission cycles.

Based on the foregoing technical solutions, in the embodiments, a device in D₂D communication receives D₂D synchronization information sent by another device, and synchronizes with a synchronization source corresponding to a synchronization source identifier carried in the D₂D synchronization information, to implement synchronization between the device and the another device; and when determining that a forwarding hop count carried in the D₂D synchronization information does not exceed a set threshold, updates the forwarding hop count in the D₂D synchronization information, and sends updated D₂D synchronization information, so that a device adjacent to the device can synchronize with the device, thereby implementing synchronization between devices in D₂D communication. In addition, complexity of communication between devices may be effectively controlled by limiting a forwarding hop count.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of a device in D₂D communication according to Embodiment 1;

FIG. 2 is a schematic structural diagram of a device in D₂D communication according to Embodiment 2;

FIG. 3 is a schematic structural diagram of a device in D₂D communication according to Embodiment 3;

FIG. 4 is a schematic structural diagram of a device in D₂D communication according to Embodiment 4;

FIG. 5a is a schematic flowchart of sending synchronization information by a device in D₂D communication according to an embodiment;

FIG. 5b is a schematic flowchart of performing synchronization by UE according to a network coverage status according to an embodiment;

FIG. 5c is a schematic diagram of a scenario of partial network coverage according to an embodiment;

FIG. 5d is a schematic diagram of synchronization between UEs outside a network coverage area according to an embodiment;

FIG. 5e is a schematic diagram of synchronization between UE outside a network coverage area and UE within the network coverage area according to an embodiment;

FIG. 6 shows an example of a synchronization process in which a PD₂DSS is used to represent a forwarding hop count according to an embodiment;

FIG. 7 is a schematic diagram of a PD₂DSCH used in data transmission and a D₂D synchronization signal;

FIG. 8a is a schematic structural diagram of an existing D₂D synchronization signal;

FIG. 8b is a schematic structural diagram in which two D₂D synchronization signal transmission frames are configured in each T₀ cycle;

FIG. 8c is a schematic structural diagram in which a total of two independent transmission windows are included in each D₂D synchronization signal transmission frame;

FIG. 8d is a schematic structural diagram in which a transmission frame for sending a D₂D synchronization signal is used as a silent period for receiving;

FIG. 9 is a schematic diagram of a device synchronization method in D₂D communication according to Embodiment 6; and

FIG. 10 shows an example of synchronously receiving multiple D₂D synchronization signals according to an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

To make the objectives, technical solutions, and advantages of the present invention clearer, the following further describes the present invention in detail with reference to the accompanying drawings. Apparently, the described embodiments are merely some but not all of the embodiments of the present invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

A core idea of implementing synchronization between devices in D₂D communication in the embodiments is as follows. Some devices in D₂D communication, serving as synchronization sources, encapsulate and send D₂D synchronization information; another device adjacent to the devices that serve as the synchronization sources receives the D₂D synchronization information, synchronizes with the synchronization sources indicated by the D₂D synchronization information, and when a forwarding hop count in the D₂D synchronization information indicating the synchronization sources does not reach a set threshold, forwards, to another adjacent device, the D₂D synchronization information indicating the synchronization sources, to implement synchronization between devices.

The following embodiments are described by using an example in which a primary D₂D synchronization sequence (PD₂DSS) carried in a synchronization signal is generated using a same generation method as that of a primary synchronization sequence (PSS) in a synchronization channel in an LTE network, and a secondary D₂D synchronization sequence (SD₂DSS) is generated using a same generation method as that of a secondary synchronization sequence (SSS) in the synchronization channel in the LTE network. In the embodiments, the PSS may be used as the PD₂DSS and the SSS may be used as the SD₂DSS, or the generation methods of the PD₂DSS and the SD₂DSS may be redefined, or the PD₂DSS and the SD₂DSS may be generated in another existing sequence generation manner. In short, the protection scope of the present invention is not limited by generation manners of the PD₂DSS and the SD₂DSS.

In Embodiment 1, as shown in FIG. 1, a device in D₂D communication is provided. The device includes a receiving unit 101, a processing unit 102, and a sending unit 103.

The receiving unit 101 receives first D₂D synchronization information sent by a first device in D₂D communication, where the first D₂D synchronization information includes a synchronization source identifier and a forwarding hop count.

The processing unit 102 determines a synchronization source according to the synchronization source identifier in the first D₂D synchronization information received by the receiving unit 101; synchronizes with the synchronization source; and updates the forwarding hop count carried in the first D₂D synchronization information when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, so as to obtain second D₂D synchronization information.

The sending unit 103 sends the second D₂D synchronization information to each adjacent device.

Preferably, the first D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device; or the first D₂D synchronization information includes first-part D₂D synchronization information and second-part D₂D synchronization information, where the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel (PD₂DSCH).

In specific implementation, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH may be the same or may be different.

The D₂D synchronization signal may include a primary D₂D synchronization signal (PD₂DSS) or a secondary D₂D synchronization signal (SD₂DSS), or may include both a PD₂DSS and an SD₂DSS.

In this embodiment, there may be the following several optional implementation manners according to different manners in which the first D₂D synchronization information carries the synchronization source identifier and the forwarding hop count.

Manner 1: The processing unit is specifically configured to determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit. The processing unit is also configured to, when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; and after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, use a D₂D synchronization signal with the replaced PD₂DSS as the second D₂D synchronization information.

Manner 2: The processing unit is specifically configured to determine a synchronization source identifier according to indication information, received by the receiving unit based on the PD₂DSCH, of the synchronization source identifier; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit. The processing unit is also configured to, when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; and after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, use a D₂D synchronization signal obtained after PD₂DSS replacement as first-part D₂D synchronization information of the second D₂D synchronization information, and use the indication information of the synchronization source identifier as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

Manner 3: The processing unit is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine a forwarding hop count according to indication information, received by the receiving unit based on the PD₂DSCH, of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and use the D₂D synchronization signal that carries the SD₂DSS as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

The sending unit is specifically configured to: send the first-part D₂D synchronization information of the second D₂D synchronization information, and send, based on the PD₂DSCH, the second-part D₂D synchronization information of the second D₂D synchronization information.

Manner 4: The processing unit is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, a synchronization source identifier and a forwarding hop count that are corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to, when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the synchronization source identifier and the current forwarding hop count; replace the SD₂DSS in the D₂D synchronization signal with the determined SD₂DSS; and use a D₂D synchronization signal obtained after SD₂DSS replacement as the second D₂D synchronization information.

The preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

Manner 5: The processing unit is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a synchronization source identifier that is corresponding to a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and synchronize with a synchronization source corresponding to the determined synchronization source identifier. The processing unit is also configured to, determine a forwarding hop count according to indication information, received by the receiving unit based on the PD₂DSCH, of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and use the D₂D synchronization signal as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

The sending unit is specifically configured to: send the first-part D₂D synchronization information of the second D₂D synchronization information, and send, based on the PD₂DSCH, the second-part D₂D synchronization information of the second D₂D synchronization information.

The preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) , represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Manner 6: The processing unit is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a synchronization source identifier that is corresponding to a PD₂DSS-SD₂DSS group to which a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit belongs; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine, according to a preset correspondence between an SD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the SD₂DSS in the combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization, and received by the receiving unit; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; acquire, according to the preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the current forwarding hop count; determine a combination of a PD₂DSS and an SD₂DSS that is corresponding to the acquired SD₂DSS and that is included in the PD₂DSS-SD₂DSS group to which the combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs; replace the combination of the PD₂DSS and the SD₂DSS in the D₂D synchronization signal with the determined combination of the PD₂DSS and the SD₂DSS; and use a D₂D synchronization signal obtained after replacement of the combination of the PD₂DSS and the SD₂DSS as the second D₂D synchronization information.

A number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

In a specific embodiment, a device in D₂D communication may simultaneously receive multiple pieces of first D₂D synchronization information. In this case, one piece of the first D₂D synchronization information needs to be selected according to a preset selection rule and synchronization is performed.

The first D₂D synchronization information further carries a synchronization source type. In this embodiment, the synchronization source type may be a network, a device, a global positioning system (GPS), or the like.

When multiple pieces of first D₂D synchronization information are received, there are the following two processing manners according to synchronization source types carried in the pieces of first D₂D synchronization information:

Manner 1: Multiple pieces of first D₂D synchronization information are received, and it is determined, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information, that the multiple pieces of first D₂D synchronization information are of a same synchronization source type. For example, it is determined, according to the synchronization source types, that the pieces of first D₂D synchronization information come from different devices in D₂D communication, or it is determined, according to the synchronization source types, that the pieces of first D₂D synchronization information come from a network side.

Preferably, performing selection according to forwarding hop counts carried in the first D₂D synchronization information includes: when determining that the receiving unit receives more than one piece of first D₂D synchronization information, separately acquiring, by the processing unit, forwarding hop counts carried in the received pieces of first D₂D synchronization information; determining a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronizing with a synchronization source corresponding to a synchronization source identifier in first D₂D synchronization information corresponding to the minimum value.

Further, when the minimum value is corresponding to more than one piece of first D₂D synchronization information, the processing unit determines signal strengths of the pieces of first D₂D synchronization information corresponding to the minimum value; and performs synchronization according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Similarly, in this specific embodiment, if determining that the forwarding hop counts carried in the pieces of first D₂D synchronization information that are received by the receiving unit are the same and the forwarding hop counts are less than the set threshold, the processing unit determines signal strengths of the pieces of first D₂D synchronization information, and performs synchronization according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Preferably, in this specific embodiment, after the forwarding hop counts carried in the pieces of first D₂D synchronization information are acquired, if determining that the minimum value of the forwarding hop counts is not less than the set threshold, the processing unit discards all the first D₂D synchronization information. Optionally, the receiving unit receives, within set duration, first D₂D synchronization information sent by another device. If determining that the receiving unit has not received the first D₂D synchronization information within the set duration, the processing unit generates the second D₂D synchronization information by using the device as a synchronization source and an identifier of the device as a synchronization source identifier and by setting the current forwarding hop count to an initial value, and instructs the sending unit to send the second D₂D synchronization information, so that the another adjacent device and the device are synchronized in time and frequency.

Manner 2: Multiple pieces of first D₂D synchronization information are received, and it is determined, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information, that the multiple pieces of first D₂D synchronization information are of different synchronization source types. For example, it is determined, according to the synchronization source types, that one piece of the first D₂D synchronization information comes from a network side, or that the other pieces of the first D₂D synchronization information come from different devices in D₂D communication respectively.

Preferably, when determining that the receiving unit receives more than one piece of first D₂D synchronization information, the processing unit determines, according to synchronization source types carried in the pieces of first D₂D synchronization information, first D₂D synchronization information in which a synchronization source is a network, and performs synchronization according to the determined first D₂D synchronization information.

Preferably, after determining the first D₂D synchronization information in which the synchronization source is a network, the processing unit acquires a forwarding hop count carried in the determined first D₂D synchronization information in which the synchronization source is a network; and if determining that the forwarding hop count is less than the set threshold, performs synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network; or if determining that the forwarding hop count is not less than the set threshold, selects one piece of the first D₂D synchronization information from the other pieces of the first D₂D synchronization information in the foregoing first processing manner and performs synchronization.

The preset threshold is set in advance. In actual application, the preset threshold may be determined according to synchronization precision on a network side and a reduction value of synchronization precision caused by each time of forwarding (a forwarding hop count of one) between devices, that is, it is ensured that synchronization precision corresponding to the preset threshold is higher than a threshold, so as to ensure synchronization precision between devices in D₂D communication, and also to avoid a reduction of stability caused when excessive devices synchronize with a same synchronization source.

Optionally, after any device in D₂D communication receives the first D₂D synchronization information in which the synchronization source type is a network, if the device has synchronized with another synchronization source type (for example, another device in D₂D communication) before, the device turns to synchronize with the network. Preferably, after the device turns to synchronize with the network, a greatest value of the forwarding hop count is reset, and the first D₂D synchronization information in which the synchronization source type is a network is forwarded according to a changed forwarding hop count.

Specifically, after performing synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network, the processing unit updates a greatest value of the forwarding hop counts, where a greatest value of the forwarding hop counts that is obtained after the update is greater than the greatest value of the forwarding hop counts that exists before the update; and when determining that the current forwarding hop count is less than the greatest value of the forwarding hop counts that is obtained after the update, updates the forwarding hop count in the first D₂D synchronization information, to obtain the second D₂D synchronization information. The sending unit sends the second D₂D synchronization information, and the sending unit sends the greatest value of the forwarding hop counts that is obtained after the update.

In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource. To avoid that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used:

Manner 1: When the receiving unit receives a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, the sending unit transmits a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: When the receiving unit receives a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, the sending unit sends a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

Manner 3: After determining that a D₂D synchronization signal needs to be sent, the sending unit sends only the D₂D synchronization signal within a set quantity of transmission cycles; and the receiving unit does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

In Embodiment 2, as shown in FIG. 2, a device of another structure in D₂D communication is provided, where the device includes a transceiver 201 and a processor 202.

The transceiver 201 receives first D₂D synchronization information sent by a first device in D₂D communication, where the first D₂D synchronization information includes a synchronization source identifier and a forwarding hop count.

The processor 202 determines a synchronization source according to the synchronization source identifier in the first D₂D synchronization information received by the transceiver 201; synchronizes with the synchronization source, when determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold; updates the forwarding hop count carried in the first D₂D synchronization information, so as to obtain second D₂D synchronization information; and instructs the transceiver 201 to send the second D₂D synchronization information.

Preferably, the first D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device; or the first D₂D synchronization information includes first-part D₂D synchronization information and second-part D₂D synchronization information, where the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel (PD₂DSCH).

In specific implementation, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH may be the same or may be different.

The D₂D synchronization signal may include a primary D₂D synchronization signal or a secondary D₂D synchronization signal (SD₂DSS), or may include both a PD₂DSS and an SD₂DSS.

In this embodiment, there may be the following several optional implementation manners according to different manners in which the synchronization information carries the synchronization source identifier and the forwarding hop count.

Manner 1: The processor 202 is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201; and when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, use a D₂D synchronization signal with the replaced PD₂DSS as the second D₂D synchronization information; and instruct the transceiver 201 to send the second D₂D synchronization information.

Manner 2: The processor 202 is specifically configured to: determine a synchronization source identifier according to indication information, received by the transceiver 201 based on the PD₂DSCH, of the synchronization source identifier; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; determine, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the PD₂DSS, carried in the D₂D synchronization signal, and received by the receiving unit; and when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the current forwarding hop count; after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, use a D₂D synchronization signal obtained after PD₂DSS replacement as first-part D₂D synchronization information of the second D₂D synchronization information, and use the indication information of the synchronization source identifier as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information; and instruct the transceiver 201 to send the second D₂D synchronization information.

Manner 3: The processor 202 is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, a synchronization source identifier that is corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine a forwarding hop count according to indication information, received by the transceiver 201 based on the PD₂DSCH, of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; use the D₂D synchronization signal that carries the SD₂DSS as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information; and instruct the transceiver to send the second D₂D synchronization information.

Manner 4: The processor 202 is specifically configured to: determine, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, a synchronization source identifier and a forwarding hop count that are corresponding to the SD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; determine, according to the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the synchronization source identifier and the current forwarding hop count; replace the SD₂DSS in the D₂D synchronization signal with the determined SD₂DSS; use a D₂D synchronization signal obtained after SD₂DSS replacement as the second D₂D synchronization information; and instruct the transceiver 201 to send the second D₂D synchronization information.

The preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

Manner 5: The processor 202 is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a synchronization source identifier that is corresponding to a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine a forwarding hop count according to indication information, received by the transceiver based on the PD₂DSCH, of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; and use the D₂D synchronization signal as first-part D₂D synchronization information of the second D₂D synchronization information, and use indication information of the current forwarding hop count as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information; and instruct the transceiver 201 to send the second D₂D synchronization information.

The preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) , represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Manner 6: The processor 202 is specifically configured to: determine, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a synchronization source identifier that is corresponding to a PD₂DSS-SD₂DSS group to which a combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization signal, and received by the transceiver 201 belongs; and synchronize with a synchronization source corresponding to the determined synchronization source identifier; and determine, according to a preset correspondence between an SD₂DSS and a forwarding hop count, a forwarding hop count that is corresponding to the SD₂DSS in the combination of the PD₂DSS and the SD₂DSS, carried in the D₂D synchronization, and received by the transceiver 201; when the determined forwarding hop count is less than the set threshold, use a sum of the determined forwarding hop count and a set value as a current forwarding hop count; acquire, according to the preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the current forwarding hop count; determine a combination of a PD₂DSS and an SD₂DSS that is corresponding to the acquired SD₂DSS and that is included in the PD₂DSS-SD₂DSS group to which the combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs; replace the combination of the PD₂DSS and the SD₂DSS in the D₂D synchronization signal with the determined combination of the PD₂DSS and the SD₂DSS; use a D₂D synchronization signal obtained after replacement of the combination of the PD₂DSS and the SD₂DSS as the second D₂D synchronization information; and instruct the transceiver 201 to send the second D₂D synchronization information.

A number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

In a specific embodiment, a device in D₂D communication may simultaneously receive multiple pieces of first D₂D synchronization information. In this case, one piece of the first D₂D synchronization information needs to be selected according to a preset selection rule and synchronization is performed.

When multiple pieces of first D₂D synchronization information are received, there are the following two processing manners according to synchronization source types carried in the pieces of first D₂D synchronization information:

Manner 1: The transceiver 201 receives multiple pieces of first D₂D synchronization information; the processor 202 determines, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information that are received by the transceiver 201, that the multiple pieces of first D₂D synchronization information are of a same synchronization source type. For example, the processor 202 determines, according to the synchronization source types, that the pieces of first D₂D synchronization information come from different devices in D₂D communication, or determines, according to the synchronization source types, that the pieces of first D₂D synchronization information come from a network side.

Preferably, performing selection according to forwarding hop counts carried in the first D₂D synchronization information includes: when determining that the transceiver 201 receives more than one piece of first D₂D synchronization information, separately acquiring, by the processor 202, forwarding hop counts carried in the received pieces of first D₂D synchronization information; determining a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronizing with a synchronization source corresponding to a synchronization source identifier in first D₂D synchronization information corresponding to the minimum value.

Further, when the minimum value is corresponding to more than one piece of first D₂D synchronization information, the processor 202 determines signal strengths of the pieces of first D₂D synchronization information corresponding to the minimum value; and performs synchronization according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Similarly, in this specific embodiment, if determining that the forwarding hop counts carried in the pieces of first D₂D synchronization information that are received by the transceiver 201 are the same and the forwarding hop counts are less than the set threshold, the processor 202 determines signal strengths of the pieces of first D₂D synchronization information, and performs synchronization according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Preferably, in this specific embodiment, after the forwarding hop counts carried in the pieces of first D₂D synchronization information are acquired, if determining that the minimum value of the forwarding hop counts is not less than the set threshold, the processor 202 discards all the first D₂D synchronization information. Optionally, the transceiver 201 receives, within set duration, first D₂D synchronization information sent by another device. If determining that the receiving unit has not received the first D₂D synchronization information within the set duration, the processor 202 generates the second D₂D synchronization information by using the device as a synchronization source and an identifier of the device as a synchronization source identifier and by setting the current forwarding hop count to an initial value, and instructs the sending unit to send the second D₂D synchronization information, so that the another adjacent device and the device are synchronized in time and frequency.

Manner 2: The transceiver 201 receives multiple pieces of first D₂D synchronization information; the processor 202 determines, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information that are received by the transceiver 201, that the multiple pieces of first D₂D synchronization information are of different synchronization source types. For example, the processor 202 determines, according to the synchronization source types, that one piece of the first D₂D synchronization information comes from a network side, or that the other pieces of the first D₂D synchronization information come from different devices in D₂D communication respectively.

Preferably, when determining that the transceiver 201 receives more than one piece of first D₂D synchronization information, the processor 202 determines, according to synchronization source types carried in the pieces of first D₂D synchronization information, first D₂D synchronization information in which a synchronization source is a network, and performs synchronization according to the determined first D₂D synchronization information.

Preferably, after determining the first D₂D synchronization information in which the synchronization source is a network, the processor 202 acquires a forwarding hop count carried in the determined first D₂D synchronization information in which the synchronization source is a network; and if determining that the forwarding hop count is less than the set threshold, performs synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network; or if determining that the forwarding hop count is not less than the set threshold, selects one piece of the first D₂D synchronization information from the other pieces of the first D₂D synchronization information in the foregoing first processing manner and performs synchronization.

The preset threshold is set in advance. In actual application, the preset threshold may be determined according to synchronization precision on a network side and a reduction value of synchronization precision caused by each time of forwarding (a forwarding hop count of one) between devices, that is, it is ensured that synchronization precision corresponding to the preset threshold is higher than a threshold, so as to ensure synchronization precision between devices in D₂D communication, and also to avoid a reduction of stability caused when excessive devices synchronize with a same synchronization source.

Optionally, after a device in D₂D communication receives the first D₂D synchronization information in which the synchronization source type is a network, if the device has synchronized with another synchronization source type (for example, another device in D₂D communication) before, the device turns to synchronize with the network. Preferably, after the device turns to synchronize with the network, a greatest value of the forwarding hop count is reset, and the first D₂D synchronization information in which the synchronization source type is a network is forwarded according to a changed forwarding hop count.

Specifically, after performing synchronization according to the determined first D₂D synchronization information in which the synchronization source is a network, the processor 202 updates a greatest value of the forwarding hop counts, where a greatest value of the forwarding hop counts that is obtained after the update is greater than the greatest value of the forwarding hop counts that exists before the update; when determining that the forwarding hop count carried in the first D₂D synchronization information is less than the greatest value of the forwarding hop counts that is obtained after the update, updates the forwarding hop count in the first D₂D synchronization information, to obtain the second D₂D synchronization information; and instructs the transceiver 201 to send the second D₂D synchronization information and instructs the transceiver 201 to send the greatest value of the forwarding hop counts that is obtained after the update.

In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource. To avoid that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used:

Manner 1: When receiving a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, the transceiver 201 transmits a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: When receiving a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, the transceiver 201 sends a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

Manner 3: After determining that a D₂D synchronization signal needs to be sent, the transceiver 201 sends only the D₂D synchronization signal within a set quantity of transmission cycles, and the transceiver 201 does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

In Embodiment 3, as shown in FIG. 3, another device in D₂D communication is provided. The device includes: a message processing unit 301, configured to determine a synchronization source identifier and a forwarding hop count; and encapsulate the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information. The device also includes a sending unit 302, configured to send the first D₂D synchronization information processed by the message processing unit 301.

If the device serves as a synchronization source, the synchronization source identifier determined by the message processing unit 301 is an identifier of the device. If a network device (for example, a base station) serves as a synchronization source, the synchronization source identifier determined by the message processing unit 301 is an identifier of the network device with which the device synchronizes.

Preferably, the first D₂D synchronization information further includes a synchronization source type.

Preferably, the first device further includes a detection unit 303 and a receiving unit 304.

The detection unit 303 is configured to detect a downlink synchronization signal sent by a network.

When determining that the detection unit 303 has not detected the downlink synchronization signal, the message processing unit 301 instructs the sending unit to send the first D₂D synchronization information; or after determining that the detection unit 303 has detected the downlink synchronization signal, determines that the receiving unit 304 has received third D₂D synchronization information sent by another device; determines, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device; and instructs the sending unit to send the first D₂D synchronization information.

In specific implementation, the first D₂D synchronization information sent by the sending unit 302 may be different from or the same as a time-frequency resource on which the downlink synchronization signal that is sent by the network and that is received by the receiving unit 303 is located.

Optionally, the sending unit 302 is specifically configured to: send a D₂D synchronization signal, where the D₂D synchronization signal carries the first D₂D synchronization information; or send a D₂D synchronization signal, where the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information; and send second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel PD₂DSCH, where the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the first D₂D synchronization information.

In actual application, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH may be different or may be the same.

Preferably, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

Corresponding to Embodiment 1, in this embodiment, there may be the following several optional implementation manners according to different manners in which the synchronization information carries the synchronization source identifier and the forwarding hop count.

Manner 1: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; add the acquired PD₂DSS and SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal that carries the PD₂DSS and the SD₂DSS as the first D₂D synchronization information.

Manner 2: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; add the acquired PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal that carries the PD₂DSS as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the determined synchronization source identifier as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

Manner 3: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; and add the acquired SD₂DSS and a preset PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information, where PD₂DSSes preset for all synchronization sources are the same in D₂D communication.

Manner 4: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count; and add the SD₂DSS to the D₂D synchronization signal, and use the D₂D synchronization signal as the first D₂D synchronization information.

Specifically, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

Manner 5: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a combination of a PD₂DSS and an SD₂DSS that corresponds to the determined synchronization source identifier; and add the acquired combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

Specifically, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Manner 6: The message processing unit 301 is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier, where each PD₂DSS-SD₂DSS group includes multiple combinations of a PD₂DSS and an SD₂DSS; acquire, according to a preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined forwarding hop count; and determine a combination of a PD₂DSS and an SD₂DSS that includes the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group; and add the determined combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal as the first D₂D synchronization information.

Preferably, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

Preferably, to avoid that in D₂D communication, multiple different devices serving as synchronization sources add a same synchronization source identifier to the first D₂D synchronization information, in this embodiment, the processing unit is further configured to: convert, according to a set cycle, the synchronization source identifier carried in the first D₂D synchronization information.

According to different manners in which the first D₂D synchronization information carries the synchronization source identifier, a purpose of converting the synchronization source identifier carried in the synchronization information may be achieved in the following specific changing manner, including: converting, by the processing unit according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal, and changing, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal.

In a specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI+Nframe(m),Ns), where N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, Nframe(m) represents a number of a radio frame on which the device is located when the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal is changed for the m^(th) time, and N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources.

In another specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI×(N_(ID) ⁽²⁾(m-1)+a),Ns), where when m is equal to 0, N_(ID) ⁽²⁾(0)=mod(IMSI+Nframe(0),Ns), N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources, a represents a non-zero integer, and Nframe(0) represents a number of a radio frame on which the device is located when the D₂D synchronization signal is sent for the first time.

The PD₂DSS and/or SD₂DSS that is used for indicating the synchronization source identifier and that is in the D₂D synchronization signal is converted, which may avoid a problem that multiple adjacent synchronization sources carry a same synchronization source identifier, so as to avoid a co-channel interference and to facilitate detection on adjacent synchronization source identifiers.

In this embodiment, when the device serves as a synchronization source, when the first D₂D synchronization information is sent, a downlink synchronization signal from a network may be received, or a synchronization signal from another synchronization source may be received. In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource. To avoid that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used:

Manner 1: The sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, transmit a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: The sending unit is specifically configured to: when the receiving unit receives a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, send a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

Manner 3: The sending unit is specifically configured to: after determining that a D₂D synchronization signal needs to be sent, send only the D₂D synchronization signal within a set quantity of transmission cycles, where the receiving unit does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

In Embodiment 4, as shown in FIG. 4, a device of another structure in D₂D communication is provided, where the device includes a processor 401 and a transceiver 402.

The processor 401 determines a synchronization source identifier and a forwarding hop count, and encapsulates the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information.

The transceiver 402 sends the first D₂D synchronization information processed by the processor 401.

Preferably, the first D₂D synchronization information further includes a synchronization source type.

The transceiver 402 is further configured to detect a downlink synchronization signal sent by a network.

When determining that the transceiver 402 has not detected the downlink synchronization signal, the processor 401 instructs the transceiver 402 to send the first D₂D synchronization information; or after determining that the transceiver 402 has detected the downlink synchronization signal, determines that the transceiver 402 has received third D₂D synchronization information sent by another device; determines, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device; and instructs the transceiver 402 to send the first D₂D synchronization information.

In specific implementation, the first D₂D synchronization information sent by the transceiver 402 may be different from or the same as a time-frequency resource on which the received downlink synchronization signal sent by the network is located.

Optionally, the transceiver 402 is specifically configured to: send a D₂D synchronization signal, where the D₂D synchronization signal carries the first D₂D synchronization information; or send a D₂D synchronization signal, where the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information; and send second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel PD₂DSCH, where the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the D₂D synchronization information.

In actual application, a cycle of the D₂D synchronization signal and a cycle of the PD₂DSCH may be different or may be the same.

Preferably, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

Corresponding to Embodiment 1 and Embodiment 2, in this embodiment, there may be the following several optional implementation manners according to different manners in which the synchronization information carries the synchronization source identifier and the forwarding hop count:

Manner 1: The processor 401 is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; add the acquired PD₂DSS and SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal that carries the PD₂DSS and the SD₂DSS as the first D₂D synchronization information.

Optionally, the device further includes a memory, configured to store the preset correspondence between a PD₂DSS and a forwarding hop count. The processor acquires the correspondence from the memory.

Manner 2: The processor 401 is specifically configured to: acquire, according to a preset correspondence between a PD₂DSS and a forwarding hop count, a PD₂DSS corresponding to the determined forwarding hop count; add the acquired PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal that carries the PD₂DSS as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the determined synchronization source identifier as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

Optionally, the device further includes a memory, configured to store the preset correspondence between a PD₂DSS and a forwarding hop count. The processor acquires the correspondence from the memory.

Manner 3: The processor 401 is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier, an SD₂DSS corresponding to the determined synchronization source identifier; and add the acquired SD₂DSS and a preset PD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information, where PD₂DSSes preset for all synchronization sources are the same in D₂D communication.

Optionally, the device further includes a memory, configured to store the preset correspondence between an SD₂DSS and a synchronization source identifier. The processor acquires the correspondence from the memory.

Manner 4: The processor 401 is specifically configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count; and add the SD₂DSS to the D₂D synchronization signal, and use the D₂D synchronization signal as the first D₂D synchronization information.

Specifically, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

Optionally, the device further includes a memory, configured to store the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count. The processor acquires the correspondence from the memory.

Manner 5: The processor 401 is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS, a combination of a PD₂DSS and an SD₂DSS that corresponds to the determined synchronization source identifier; and add the acquired combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; use the D₂D synchronization signal as the first-part D₂D synchronization information of the first D₂D synchronization information; and use indication information of the forwarding hop count as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

Specifically, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Optionally, the device further includes a memory, configured to store the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS. The processor acquires the correspondence from the memory.

Manner 6: The processor 4o1 is specifically configured to: acquire, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier, where each PD₂DSS-SD₂DSS group includes multiple combinations of a PD₂DSS and an SD₂DSS; acquire, according to a preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined forwarding hop count; and determine a combination of a PD₂DSS and an SD₂DSS that includes the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group; and add the determined combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal; and use the D₂D synchronization signal as the first D₂D synchronization information.

Preferably, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

Optionally, the device further includes a memory, configured to store the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, and store the preset correspondence between an SD₂DSS and a forwarding hop count. The processor acquires the correspondences from the memory.

Preferably, to avoid that in D₂D communication, multiple different devices serving as synchronization sources add a same synchronization source identifier to the first D₂D synchronization information, in this embodiment, the processor is further configured to: convert, according to a set cycle, the synchronization source identifier carried in the first D₂D synchronization information.

According to different manners in which the first D₂D synchronization information carries the synchronization source identifier, a purpose of converting the synchronization source identifier carried in the synchronization information may be achieved in the following specific changing manner, including: converting, by the processor according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal, and changing, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal.

In a specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI+Nframe(m)Ns), where

N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, Nframe(m) represents a number of a radio frame on which the device is located when the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal is changed for the m^(th) time, and N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources.

In another specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is:

in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI×(N_(ID) ⁽²⁾(m-1)+a),Ns), where

when m is equal to 0, N_(ID) ⁽²⁾(0)=mod(IMSI+Nframe(0),Ns), N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources, a represents a non-zero integer, and Nframe(0) represents a number of a radio frame on which the device is located when the D₂D synchronization signal is sent for the first time.

The PD₂DSS and/or SD₂DSS that is used for indicating the synchronization source identifier and that is in the D₂D synchronization signal is converted, which may avoid a problem that multiple adjacent synchronization sources carry a same synchronization source identifier, so as to avoid a co-channel interference and to facilitate detection on adjacent synchronization source identifiers.

In this embodiment, when the device serves as a synchronization source, when the first D₂D synchronization information is sent, a downlink synchronization signal from a network may be received, or a synchronization signal from another synchronization source may be received. In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource. To avoid that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used:

Manner 1: When receiving a D₂D synchronization signal by using any one of radio frames used for sending a D₂D synchronization signal, the transceiver transmits a D₂D synchronization signal by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: When receiving a D₂D synchronization signal by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, the transceiver sends a D₂D synchronization signal by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

Manner 3: After determining that a D₂D synchronization signal needs to be sent, the transceiver sends only the D₂D synchronization signal within a set quantity of transmission cycles, and does not receive any D₂D synchronization signal within the set quantity of transmission cycles.

In Embodiment 5, based on a same principle, a method for sending synchronization information by a device in D₂D communication is provided. As shown in FIG. 5 a, the method mainly includes the following steps.

Step 51: A first device in D₂D communication determines a synchronization source identifier and a forwarding hop count, and encapsulates the determined synchronization source identifier and forwarding hop count into first D₂D synchronization information.

Step 52: Send the first D₂D synchronization information, where a second device in D₂D communication receives the first D₂D synchronization information; synchronizes with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information; and when determining that the forwarding hop count in the first D₂D synchronization information is less than the set threshold, updates the forwarding hop count in the first D₂D synchronization information and sends second D₂D synchronization information obtained after update of the forwarding hop count.

The first device in D₂D communication is a device serving as a synchronization source.

Optionally, the first D₂D synchronization information further includes a synchronization source type.

Preferably, in this embodiment, when a device serving as a synchronization source is outside a network coverage area, the device generates and sends first D₂D synchronization information, where a determined synchronization source identifier carried in the first D₂D synchronization information is an identifier of the device serving as a synchronization source. If the device serving as a synchronization source is within the network coverage area, the device first synchronizes with a network; when determining that another adjacent device that is outside the network coverage area needs to be synchronized, the device sends first D₂D synchronization information, where a determined synchronization source identifier carried in the first D₂D synchronization information is an identifier of the network device with which the device serving as a synchronization source synchronizes.

Specifically, before sending the first D₂D synchronization information, the first device in D₂D communication detects a downlink synchronization signal sent by a network, and determines that the downlink synchronization signal is not detected; or the first device detects a downlink synchronization signal sent by a network; after determining that the downlink synchronization signal is detected, receives third D₂D synchronization information sent by another device; and determines, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device.

A time-frequency occupied by the first D₂D synchronization information that is sent by the first device is different from a time-frequency resource occupied by the downlink synchronization signal that is sent by the network.

For example, as shown in FIG. ₅b, a specific process in which user equipment (UE) performs synchronization according to a network coverage status is as follows.

Step 501: The UE scans a downlink synchronization signal from an LTE network.

Step 502: Determine whether the UE has detected the downlink synchronization signal; if yes, perform step 503; and if not, perform step 504.

Step 503: When the UE is within a network coverage area, if D₂D synchronization information of another device is received, and it is determined that a synchronization source of the D₂D synchronization information is not a network, send D₂D synchronization information in which a synchronization source identifier is a network.

Step 504: When the UE is outside a network coverage area, the UE scans D₂D synchronization information on a D₂D resource.

Step 505: Determine whether the UE detects the D₂D synchronization information; if yes, perform step 507; and if not, perform step 506.

Step 506: The UE serves as a synchronization source and generates D₂D synchronization information, and sends the D₂D synchronization information.

Step 507: The UE performs synchronization according to the detected D₂D synchronization information, and forwards the D₂D synchronization information.

Preferably, after receiving D₂D synchronization information in which a synchronization source type is a network, the UE outside the network coverage area synchronizes to the network according to the D₂D synchronization information instead, and changes a greatest value of a forwarding hop count, where a greatest value obtained after the change is greater than a greatest value of a forwarding hop count in the D₂D synchronization information in which the synchronization source is not a network.

For example, as shown in FIG. 5 c, in D₂D communication, some UEs (for example, UE₁) are within a network coverage area, and some UEs (UE₂, UE₃, and UE₄) are outside the network coverage area. All UEs in D₂D communication detect D₂D synchronization information on a D₂D resource. If no D₂D synchronization information from the UEs outside the network coverage area is detected, the UE₁ within the network coverage area does not send any D₂D synchronization information. The UEs outside the network coverage area periodically detect whether there is D₂D synchronization information from the network coverage area. If no D₂D synchronization information from the network coverage area is detected, as shown in FIG. 5 d, the UE₃ outside the network coverage area serves as a synchronization source, and generates and sends D₂D synchronization information to the UE₂ and the UE₄; the UE₂ forwards the D₂D synchronization information in which the synchronization source is the UE₃ to the UE₁; the UE₁ determines, according to the received D₂D synchronization information, that the synchronization source of the D₂D synchronization information is the UE outside the network coverage area, and as shown in FIG. 5e , sends D₂D synchronization information in which a synchronization source is a base station to the UE₂. When determining that the synchronization source of the D₂D synchronization information is the base station, the UE₂ synchronizes to the base station instead, and forwards the D₂D synchronization information in which the synchronization source is the base station to the UE₃ and the UE₄.

Optionally, the device sends a D₂D synchronization signal, where the D₂D synchronization signal carries the first D₂D synchronization information; or the device sends a D₂D synchronization signal, where the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information, and sends second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel PD₂DSCH, where the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the first D₂D synchronization information.

Optionally, if the first D₂D synchronization information includes the synchronization source type, indication information of the synchronization source type may be carried in the D₂D synchronization signal and sent, or may be sent through the PD₂DSCH.

In actual application, a transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH may be the same or may be different.

Optionally, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

In this embodiment, there may be the following several optional implementation manners according to different manners in which the synchronization information carries the synchronization source identifier and the forwarding hop count.

Manner 1: A PD₂DSS corresponding to the determined forwarding hop count is acquired according to a preset correspondence between a PD₂DSS and a forwarding hop count; an SD₂DSS corresponding to the determined synchronization source identifier is acquired according to a preset correspondence between an SD₂DSS and a synchronization source identifier; the acquired PD₂DSS and SD₂DSS are added to the D₂D synchronization signal; and the D₂D synchronization signal is used as the first D₂D synchronization information.

For example, assuming that there are M PD₂DSSes, mapping relationships between forwarding hop counts for UE to forward a D₂D synchronization signal and the M PD₂DSSes are preset. A simple mapping method is: For D₂D synchronization signals from a same synchronization source, a maximum of K times of forwarding is allowed, that is, a greatest value of the forwarding hop count is K, where K is not greater than M, and the i^(th) forwarding is corresponding to the i^(th) PD₂DSS, that is, a forwarding hop count i is corresponding to the i^(th) PD₂DSS. As shown in Table 1, in the table, M is 3, that is, a quantity of PD₂DSSes is 3, which is only an example herein and is not used to impose a limitation on the present invention. Likewise, numbers of the PD₂DSSes may be 0, 1, and 2, and corresponding forwarding hop counts may also be 0, 1, and 2, which differ from the content shown in Table 1 only in starting point, but representation methods are of no difference and should be considered as a same method. Besides, only ZC sequences with root sequence numbers of 25, 29, and 34 are used as examples in the table, whereas in actual application, another ZC sequence may also be used.

TABLE 1 Forwarding hop count PD2DSS Root sequence number indicated by a PD2DSS from number of a PD2DSS a same synchronization source 1 25 1 2 29 2 3 34 3

For another example, still in the foregoing example, a specific synchronization process is as follows: Assuming that UE₁ detects a D₂D synchronization signal, and that the D₂D synchronization signal carries the i^(th) PD₂DSS, the UE₁ synchronizes timing and frequency of a receiver of the UE₁ to a synchronization source indicated by the received D₂D. The UE₁ compares i and K. If i is greater than K, the UE₁ does not forward the D₂D synchronization signal of the synchronization source; if i is not greater than K, the UE₁ replaces the i^(th) PD₂DSS carried in the D₂D synchronization signal with the (i+t)^(th) PD₂DSS, and sends the D₂D synchronization signal. Assuming that the UE₁ does not detect any D₂D synchronization signal, the UE₁ sends a D₂D synchronization signal on a D₂D resource, where the D₂D synchronization signal carries the first PD₂DSS.

Still in the foregoing example, as shown in FIG. 6, assuming that the UE₁ is a synchronization source, and that a D₂D synchronization signal sent by the UE₁ carries a PD₂DSS₁, both UE₂ and UE₃ receive the D₂D synchronization signal carrying the PD₂DSS₁, and the D₂D synchronization signal of the synchronization source is forwarded once from the UE₁ to the UE₂ and the UE₃. The UE₂ and the UE₃ separately synchronize timing and frequency of their receivers to the UE₁, and the UE₂ and the UE₃, executing a second time of forwarding for the synchronization source, replace the PD₂DSS₁ in the D₂D synchronization signal with a PD₂DSS₂. UE₄ receives the D₂D synchronization signal carrying the PD₂DSS₂, synchronizes timing and frequency of a receiver of the UE₄ to the synchronization source UE₁, executes a third time of forwarding, and replaces the PD₂DSS₂ in the D₂D synchronization signal with a PD₂DSS₃. UE₅ receives the D₂D synchronization signal carrying the PD₂DSS₃, and synchronizes timing and frequency of a receiver of the UE₅ to the synchronization source UE₁. Because in this case, a maximum forwarding hop count 3 is reached, the UE₅ does not forward the D₂D synchronization signal in which the synchronization source is the UE₁. UE₆ cannot receive the D₂D synchronization signal, serves as a new synchronization source, and generates and sends a D₂D synchronization signal, where the D₂D synchronization signal carries the PD₂DSS₁. The UE₅ and UE₇ receive the D₂D synchronization signal from the UE₆ and also perform forwarding in the foregoing manner until the maximum forwarding hop count is reached.

In this implementation manner, if the SD₂DSS is represented by an SSS in LTE, there may be at least 167 different SD₂DSSes, and correspondingly, there may be at least 167 synchronization source identifiers.

In this implementation manner, when multiple D₂D synchronization signals with a same forwarding hop count are from a same synchronization source, the multiple D₂D synchronization signals may be directly combined to obtain a combination gain.

Manner 2: A PD₂DSS corresponding to the determined forwarding hop count is acquired according to a preset correspondence between a PD₂DSS and a forwarding hop count; the acquired PD₂DSS is added to the D₂D synchronization signal; the D₂D synchronization signal is used as the first-part D₂D synchronization information of the first D₂D synchronization information; and indication information of the determined synchronization source identifier is used as the second-part D₂D synchronization information that is sent based on the PD₂DSCH and that is of the first D₂D synchronization information.

For example, as shown in FIG. 7, during transmission of a D₂D synchronization signal (D₂DSS), data may be transmitted through a PD₂DSCH, where the data may include at least indication information of a synchronization source identifier, and a cycle that a PD₂DSCH signal appears is not less than a cycle that a D₂D synchronization signal appears. That is, there is no PD₂DSCH signal near some D₂D synchronization signals, but there must be a D₂D synchronization signal near a PD₂DSCH signal.

D₂D synchronization signals with a same forwarding hop count that are from a same synchronization source are the same, and content that is transmitted based on the PD₂DSCH is the same.

In this implementation manner, at a stage of selecting a synchronization source, the PD₂DSCH signal does not need to be parsed, and the PD₂DSCH signal needs to be parsed only after the synchronization source is selected. In addition, when multiple D₂D synchronization signals with a same forwarding hop count are from a same synchronization source, the multiple D₂D synchronization signals may be directly combined to obtain a combination gain.

Manner 3: An SD₂DSS corresponding to the determined synchronization source identifier is acquired according to a preset correspondence between an SD₂DSS and a synchronization source identifier; and the acquired SD₂DSS and a preset PD₂DSS are added to the D₂D synchronization signal; the D₂D synchronization signal is used as the first-part D₂D synchronization information of the first D₂D synchronization information; and indication information of the determined forwarding hop count is used as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information, where PD₂DSSes preset for all synchronization sources are the same in D₂D communication.

For example, different SD₂DSSes indicate different synchronization sources. Assuming that there are Ns SD₂DSSes in total, the Ns SD₂DSSes may indicate Ns different synchronization sources, and the indication information of the forwarding hop count is sent through the PD₂DSCH.

In this implementation manner, only one PD₂DSS is preset for all UEs and different forwarding hop counts. When UE receives a PD₂DSS, greater energy can be obtained by receiving the superimposed PD₂DSS. In addition, the UE needs to detect only one PD₂DSS, reducing detection complexity. A receiver of the UE only needs to demodulate, after finding multiple PD₂DSS synchronization locations, SD₂DSSes at the synchronization locations.

Manner 4: An SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count is acquired according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count; and the SD₂DSS is added to the D₂D synchronization signal, and the D₂D synchronization signal is used as the first D₂D synchronization information.

For example, SD₂DSSes are divided into M groups, a number of a group is corresponding to a forwarding hop count, and a sequence number of a PD₂DSS in a group is corresponding to a synchronization source identifier.

Specifically, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

For example, assuming that a sequence number of an SD₂DSS carried in a detected D₂D synchronization signal is 125, and a preset maximum value of a forwarding hop count is 3, it can be calculated that a forwarding hop count is 2 and a synchronization source identifier is 41.

In this implementation manner, only a same PD₂DSS may be preset for all UEs and forwarding hop counts, so that greater energy can be obtained by receiving the superimposed PD₂DSS. In addition, UE needs to detect only one PD₂DSS, reducing detection complexity, improving a speed for receiving a PD₂DSS, and selecting a synchronization source without a need of receiving a PD₂DSCH.

Manner 5: A combination of a PD₂DSS and an SD₂DSS that corresponds to the determined synchronization source identifier is acquired according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS; and the acquired combination of a PD₂DSS and an SD₂DSS is added to the D₂D synchronization signal; the D₂D synchronization signal is used as the first-part D₂D synchronization information of the first D₂D synchronization information; and indication information of the determined forwarding hop count is used as the second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the first D₂D synchronization information.

Specifically, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID)(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Manner 6: A PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier is acquired according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, where each PD₂DSS-SD₂DSS group includes multiple combinations of a PD₂DSS and an SD₂DSS; an SD₂DSS corresponding to the determined forwarding hop count is acquired according to a preset correspondence between an SD₂DSS and a forwarding hop count; and a combination of a PD₂DSS and an SD₂DSS that includes the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group is determined; and the determined combination of a PD₂DSS and an SD₂DSS is added to the D₂D synchronization signal; and the D₂D synchronization signal is used as the first D₂D synchronization information.

Optionally, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

For example, assuming that there are M PD₂DSSes, and after receiving a PD₂DSS of a synchronization source, UE uses the same PD₂DSS to perform forwarding, and a maximum of K times of forwarding is allowed. If UE₁ receives the m^(th) PD₂DSS and the n_(k) ^(th) SD₂DSS that are carried in the k^(th) hop of D₂D synchronization signal from UE A, the UE₁ synchronizes timing and frequency of a receiver of the UE₁ to a synchronization source indicated by the PD₂DSS. The UE₁ compares k and K. If k is greater than K, the UE₁ does not send the D₂D synchronization signal; if k is not greater than K, the UE₁ sends the D₂D synchronization signal, where the sent D₂D synchronization signal carries the m^(th) PD₂DSS and the n_(k+1) ^(th) SD₂DSS.

Preferably, in the first to the sixth implementation manners, if D₂D synchronization signals corresponding to a same synchronization source carry a same PD₂DSS, the D₂D synchronization signals that carry the same PD₂DSS and that are received within a set time may be received after combination, so as to obtain a combination gain from receiving.

Preferably, in the first to the sixth implementation manners, if data content that is transmitted by using the PD₂DSCH, that has a same forwarding hop count, and that is corresponding to a same synchronization source is the same, data content may also be combined to obtain a synchronization gain.

Preferably, to avoid that in D₂D communication, multiple different devices serving as synchronization sources add a same synchronization source identifier to the first D₂D synchronization information, in this embodiment, the synchronization source identifier carried in the first D₂D synchronization information is converted according to a set cycle.

According to different manners in which the first D₂D synchronization information carries the synchronization source identifier, a purpose of converting the synchronization source identifier carried in the synchronization information may be achieved in the following specific changing manner, including: converting, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal; or converting, according to the set cycle, the PD₂DSS carried in the D₂D synchronization signal, and changing, according to the set cycle, the SD₂DSS carried in the D₂D synchronization signal.

In a first specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI+Nframe(m),Ns), where

N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, Nframe(m) represents a number of a radio frame on which the device is located when the PD₂DSS/SD₂DSS carried in the D₂D synchronization signal is changed for the m^(th) time, and N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources.

In a second specific implementation manner, a specific method for changing the PD₂DSS and/or SD₂DSS used for indicating the synchronization source identifier is: in the m^(th) set cycle, a sequence number of the PD₂DSS/SD₂DSS is N_(ID) ⁽²⁾(m)=mod(IMSI×(N_(ID) ⁽²⁾(m-1)+a),Ns), where when m is equal to 0, N_(ID) ⁽²⁾(0)=mod(IMSI+Nframe(0),Ns), N_(ID) ⁽²⁾ represents a sequence number of the PD₂DSS/SD₂DSS or a synchronization source identifier, IMSI is an international mobile subscriber identity, N_(s) represents a maximum quantity of PD₂DSSes/SD₂DSSes or a maximum quantity of identifiable synchronization sources, a represents a non-zero integer, and Nframe(0) represents a number of a radio frame on which the device is located when the D₂D synchronization signal is sent for the first time.

The PD₂DSS and/or SD₂DSS that is used for indicating the synchronization source identifier and that is in the D₂D synchronization signal is converted, which may avoid a problem that multiple adjacent synchronization sources carry a same synchronization source identifier, so as to avoid a co-channel interference and to facilitate detection on adjacent synchronization source identifiers.

In this embodiment , when the device serves as a synchronization source, when the first D₂D synchronization information is sent, a downlink synchronization signal from a network may be received, or a synchronization signal from another synchronization source may be received. In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource.

For example, as shown in FIG. 8a , in the prior art, it is specified that at a fixed location in each long cycle T₀, a D₂D synchronization signal frame and a corresponding D₂D synchronization signal appear; if UE needs to both send and receive a D₂D synchronization signal at the fixed location, the UE cannot work on a channel used for a D₂D synchronization signal.

To avoid mutual interference caused by that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used:

Manner 1: When a D₂D synchronization signal is received by using any one of radio frames used for sending a D₂D synchronization signal, a D₂D synchronization signal is transmitted by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

For example, as shown in FIG. 8b , in each long cycle T₀, multiple radio frames used for a D₂D synchronization signal are configured. Assuming that T₀ is one second, the multiple radio frames used for a D₂D synchronization signal may be configured in too radio frames, and in each radio frame used for a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: When a D₂D synchronization signal is received by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, a D₂D synchronization signal is sent by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

For example, as shown in FIG. 8c , in each long cycle T₀, two independent D₂D synchronization signal frame sending windows are configured. When receiving a D₂D synchronization signal, UE may track both of the two sending windows, to detect the D₂D synchronization signal. After determining that the UE needs to synchronize timing to the D₂D synchronization signal of either of the sending windows, the UE also needs to send a D₂D synchronization signal. In this case, when tracking the sending window corresponding to the D₂D synchronization signal to which timing of the UE synchronizes, the UE sends another D₂D synchronization signal on the other sending window.

The radio frame used for sending a D₂D synchronization signal may be one radio frame, or may be multiple radio frames that are consecutive or inconsecutive in time.

Manner 3: After it is determined that a D₂D synchronization signal needs to be sent, only the D₂D synchronization signal is sent within a set quantity of transmission cycles, and no D₂D synchronization signal is received within the set quantity of transmission cycles.

For example, as shown in FIG. 8d , after UE receives a D₂D synchronization signal in the first T₀ cycle, a receiver of the UE does not receive data from any synchronization source in the second T₀ cycle, that is, uses the second T₀ as a silent period for receiving, and only transmits a D₂D synchronization signal. After successful transmission, the UE receives a D₂D synchronization signal in a next T₀ cycle.

In Embodiment 6, as shown in FIG. 9, a device synchronization method in D₂D communication is provided based on a same principle, where the method mainly includes the following steps.

Step 901: A second device in D₂D communication receives first D₂D synchronization information sent by a first device, where the first D₂D synchronization information includes a synchronization source identifier and a forwarding hop count.

Step 902: Synchronize with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information, and when it is determined that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, update the forwarding hop count in the first D₂D synchronization information, so as to obtain second D₂D synchronization information.

Step 903: Send the second D₂D synchronization information.

Optionally, the first D₂D synchronization information is carried in a D₂D synchronization signal sent by the another device; or the first D₂D synchronization information includes first-part D₂D synchronization information and second-part D₂D synchronization information, where the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel PD₂DSCH.

A transmission cycle of the D₂D synchronization signal and a transmission cycle of the PD₂DSCH are different.

Optionally, the D₂D synchronization signal includes a primary D₂D synchronization signal PD₂DSS and/or a secondary D₂D synchronization signal SD₂DSS.

Corresponding to Embodiment 5, in this embodiment, there may be the following several optional implementation manners according to different manners in which the synchronization information carries the synchronization source identifier and the forwarding hop count:

Manner 1: Corresponding to the first implementation manner provided in Embodiment 5, a synchronization source identifier that is corresponding to the SD₂DSS and that is carried in the received D₂D synchronization signal is determined according to a preset correspondence between an SD₂DSS and a synchronization source identifier; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier, a forwarding hop count that is corresponding to the PD₂DSS and that is carried in the received D₂D synchronization signal is determined according to a preset correspondence between a PD₂DSS and a forwarding hop count; and when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; a PD₂DSS corresponding to the current forwarding hop count is determined according to the preset correspondence between a PD₂DSS and a forwarding hop count; and after the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS, a D₂D synchronization signal with the replaced PD₂DSS is used as the second D₂D synchronization information.

Manner 2: Corresponding to the second implementation manner provided in Embodiment 5, indication information of the synchronization source identifier is received based on the PD₂DSCH; the synchronization source identifier is determined according to the indication information of the synchronization source identifier; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier; a forwarding hop count that is corresponding to the PD₂DSS and that is carried in the received D₂D synchronization signal is determined according to a preset correspondence between a PD₂DSS and a forwarding hop count; and when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; a PD₂DSS corresponding to the current forwarding hop count is determined according to the preset correspondence between a PD₂DSS and a forwarding hop count; the PD₂DSS in the received D₂D synchronization signal is replaced with the determined PD₂DSS; and a D₂D synchronization signal obtained after PD₂DSS replacement is used as first-part D₂D synchronization information of the second D₂D synchronization information, and the indication information of the synchronization source identifier is used as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

Manner 3: Corresponding to the third implementation manner provided in Embodiment 5, a synchronization source identifier that is corresponding to the SD₂DSS carried in the received D₂D synchronization signal is determined according to a preset correspondence between an SD₂DSS and a synchronization source identifier; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier; and indication information of the forwarding hop count is received based on the PD₂DSCH; the forwarding hop count is determined according to the indication information of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; and the synchronization signal that carries the SD₂DSS is used as first-part D₂D synchronization information of the second D₂D synchronization information, and indication information of the current forwarding hop count is used as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

Manner 4: Corresponding to the fourth implementation manner provided in Embodiment 5, a synchronization source identifier and a forwarding hop count that are carried in the D₂D synchronization signal and that are corresponding to the SD₂DSS are determined according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier; and when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; an SD₂DSS corresponding to the synchronization source identifier and the current forwarding hop count is determined according to the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count; the SD₂DSS in the D₂D synchronization signal is replaced with the determined SD₂DSS; and a D₂D synchronization signal obtained after SD₂DSS replacement is used as the second D₂D synchronization information.

Specifically, the preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count includes: the preset correspondence between an SD₂DSS and a forwarding hop count satisfying N_(k)=mod(N_(ID) ⁽²⁾,K), where N_(k) represents a forwarding hop count, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and mod represents a modulo operation; and the preset correspondence between an SD₂DSS and a synchronization source identifier satisfying N_(ID)=floor(N_(ID) ⁽²⁾/K), where N_(ID) represents a synchronization source identifier, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, K represents a preset maximum value of the forwarding hop count, and floor represents a rounding down operation.

Manner 5: Corresponding to the fifth implementation manner provided in Embodiment 5, a synchronization source identifier that is corresponding to a combination of the PD₂DSS and the SD₂DSS and that is carried in the received D₂D synchronization signal is determined according to a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier; and indication information of the forwarding hop count is received based on the PD₂DSCH; the forwarding hop count is determined according to the indication information of the forwarding hop count; when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; and the D₂D synchronization signal is used as first-part D₂D synchronization information of the second D₂D synchronization information, and indication information of the current forwarding hop count is used as second-part D₂D synchronization information that is sent through the PD₂DSCH and that is of the second D₂D synchronization information, to obtain the second D₂D synchronization information.

Specifically, the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS includes: the preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, and N_(s) represents a maximum quantity of SD₂DSSes.

Manner 6: Corresponding to the sixth implementation manner provided in Embodiment 5, a synchronization source identifier that is corresponding to a PD₂DSS-SD₂DSS group to which a combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs is determined according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group; and synchronization is performed with a synchronization source corresponding to the determined synchronization source identifier; and a forwarding hop count that is corresponding to the SD₂DSS in the combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal is determined according to a preset correspondence between an SD₂DSS and a forwarding hop count; when the determined forwarding hop count is less than the set threshold, a sum of the determined forwarding hop count and a set value is used as a current forwarding hop count; an SD₂DSS corresponding to the current forwarding hop count is acquired according to the preset correspondence between an SD₂DSS and a forwarding hop count; a combination of a PD₂DSS and an SD₂DSS that is corresponding to the acquired SD₂DSS and that is included in the PD₂DSS-SD₂DSS group to which the combination of the PD₂DSS and the SD₂DSS carried in the received D₂D synchronization signal belongs is determined; the combination of the PD₂DSS and the SD₂DSS in the D₂D synchronization signal is replaced with the determined combination of the PD₂DSS and the SD₂DSS; and a D₂D synchronization signal obtained after replacement of the combination of the PD₂DSS and the SD₂DSS is used as the second D₂D synchronization information.

Optionally, a number of the PD₂DSS-SD₂DSS group is used to indicate a synchronization source identifier; and a sequence number of the SD₂DSS is used to indicate a forwarding hop count in a D₂D synchronization signal corresponding to the synchronization source identifier indicated by a number of a PD₂DSS-SD₂DSS group to which the SD₂DSS belongs.

Specifically, the preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group includes: a preset correspondence between a synchronization source identifier and a combination of a PD₂DSS and an SD₂DSS satisfying N_(ID)=M_(s)N_(ID) ⁽¹⁾+N_(ID) ⁽²⁾, where N_(ID) represents a synchronization source identifier, N_(ID) ⁽¹⁾ represents a sequence number of a PD₂DSS, N_(ID) ⁽²⁾ represents a sequence number of an SD₂DSS, 0≦N_(ID) ⁽¹⁾≦(M_(s)−1), 0≦N_(ID) ⁽²⁾≦(N_(s)−1), M_(s) represents a maximum quantity of PD₂DSSes, N_(s) is obtained by rounding down a value obtained after a maximum value of the sequence number of the SD₂DSS is divided by a maximum value of a forwarding hop count; and each combination of a PD₂DSS and an SD₂DSS that corresponds to a same synchronization source identifier belongs to one PD₂DSS-SD₂DSS group, and a number of the PD₂DSS-SD₂DSS group is the corresponding synchronization source identifier.

In a specific embodiment, a device in D₂D communication may simultaneously receive multiple pieces of first D₂D synchronization information. In this case, one piece of the first D₂D synchronization information needs to be selected according to a preset selection rule and synchronization is performed.

The first D₂D synchronization information further carries a synchronization source type. Specifically, the synchronization source type may be indicated by using a D₂D synchronization signal or a PD₂DSCH.

For example, UE within a network coverage area sends a D₂D synchronization signal on a D₂D resource, and indicates, by using the PD₂DSCH, that a synchronization source type is an LTE network.

In this embodiment, the synchronization source type may be a network, a device, a global positioning system (GPS), or the like.

When multiple pieces of first D₂D synchronization information are received, there are the following two processing manners according to synchronization source types carried in the pieces of first D₂D synchronization information.

Manner 1: Multiple pieces of first D₂D synchronization information are received, and it is determined, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information, that the multiple pieces of first D₂D synchronization information are of a same synchronization source type. For example, it is determined, according to the synchronization source types, that the pieces of first D₂D synchronization information come from different devices in D₂D communication, or it is determined, according to the synchronization source types, that the pieces of first D₂D synchronization information come from a network side.

Preferably, performing selection according to forwarding hop counts carried in the first D₂D synchronization information includes: receiving the first D₂D synchronization information; when determining that more than one piece of first D₂D synchronization information is received, separately acquiring forwarding hop counts carried in the received pieces of first D₂D synchronization information; determining a minimum value of the acquired forwarding hop counts; and when determining that the minimum value is less than the set threshold, synchronizing with a synchronization source corresponding to a synchronization source identifier in first D₂D synchronization information corresponding to the minimum value.

Further, if the minimum value is corresponding to more than one piece of first D₂D synchronization information, signal strengths of the first D₂D synchronization information corresponding to the minimum value are determined, and synchronization is performed according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Similarly, in this specific embodiment, if it is determined that the forwarding hop counts carried in the pieces of received first D₂D synchronization information are the same and the forwarding hop counts are less than the set threshold, signal strengths of the pieces of first D₂D synchronization information are determined, and synchronization is performed according to first D₂D synchronization information corresponding to a greatest value of the signal strengths.

Preferably, in this specific embodiment, after the forwarding hop counts carried in the pieces of first D₂D synchronization information are acquired, if it is determined that a minimum value of the forwarding hop counts is not less than the set threshold, all the first D₂D synchronization information is discarded. Optionally, first D₂D synchronization information sent by another device is received within set duration. If it is determined that the first D₂D synchronization information is not received within the set duration, the second D₂D synchronization information is generated by using the device a synchronization source and an identifier of the device as a synchronization source identifier and by setting the current forwarding hop count to an initial value, and the second D₂D synchronization information is sent, so that other adjacent devices and the device are synchronized in time and frequency.

For example, as shown in FIG. 10, it is assumed that PD₂DSSes of D₂D synchronization signals of a same synchronization source are the same, and that a maximum forwarding hop count is 3. A D₂D synchronization signal from UE₁ carries a PD₂DSS₁; the D₂D synchronization signal is forwarded to UE₂ and UE₃ via a first hop, forwarded separately by the UE₂ and the UE₃ to UE₄ through second-hop forwarding, and forwarded by the UE₄ to UE₅ through third-hop forwarding; the UE₅ does not forward the D₂D synchronization signal. UE_(6,) as a synchronization source, sends a D₂D synchronization signal carrying the PD₂DSS₁, and the D₂D synchronization signal is forwarded to the UE₅ and UE₇ by using a first hop; if the UE₅ also forwards the D₂D synchronization signal to the UE₄ via a second hop, the UE₄ may receive the D₂D synchronization signals from two different synchronization sources, and forwarding hop counts of the two D₂D synchronization signals are the same and are both the second hop. Because the UE₄ simultaneously receives the D₂D synchronization signal that is from the synchronization source UE₁ and that is forwarded by the UE₂ and the UE₃ via the second hop, it is determined that a strength of the D₂D synchronization signal from the synchronization source UE is greater than a strength of the D₂D synchronization signal that is from the synchronization source UE₆ and that is forwarded via the second hop. Therefore, the UE₄ performs synchronization according to the D₂D synchronization signal of which the synchronization source is the UE₁.

Manner 2: Multiple pieces of first D₂D synchronization information are received, and it is determined, according to synchronization source types carried in the multiple pieces of first D₂D synchronization information, that the multiple pieces of first D₂D synchronization information are of different synchronization source types. For example, it is determined, according to the synchronization source types, that one piece of the first D₂D synchronization information comes from a network side, or that the other pieces of the first D₂D synchronization information come from different devices in D₂D communication respectively.

Preferably, when it is determined that more than one piece of first D₂D synchronization information is received, first D₂D synchronization information in which a synchronization source is a network is determined according to synchronization source types carried in the D₂D synchronization signals, and synchronization is performed according to the determined first D₂D synchronization information.

Preferably, after the first D₂D synchronization information in which the synchronization source is a network is determined, a forwarding hop count carried in the determined first D₂D synchronization information in which the synchronization source is a network is acquired; and if it is determined that the forwarding hop count is less than the set threshold, synchronization is performed according to the determined first D₂D synchronization information in which the synchronization source is a network; or if it is determined that the forwarding hop count is not less than the set threshold, one piece of the first D₂D synchronization information is selected from the other pieces of the first D₂D synchronization information in the foregoing first processing manner and synchronization is performed.

The preset threshold is set in advance. In actual application, the preset threshold may be determined according to synchronization precision on a network side and a reduction value of synchronization precision caused by each time of forwarding (a forwarding hop count of one) between devices, that is, it is ensured that synchronization precision corresponding to the preset threshold is higher than a threshold, so as to ensure synchronization precision between devices in D₂D communication, and also to avoid a reduction of stability caused when excessive devices synchronize with a same synchronization source.

Optionally, after the second device in D₂D communication receives the D₂D synchronization information in which the synchronization source type is a network, if the second device has synchronized with another synchronization source type (for example, another device in D₂D communication) before, the device turns to synchronize with the network. Preferably, after the device turns to synchronize with the network, a greatest value of the forwarding hop count is reset, and the D₂D synchronization information in which the synchronization source type is a network is forwarded according to a changed forwarding hop count.

Specifically, after synchronization is performed according to the determined first D₂D synchronization information in which the synchronization source is a network, a greatest value of the forwarding hop counts is updated, where a greatest value of the forwarding hop counts that is obtained after the update is greater than the greatest value of the forwarding hop counts that exists before the update. When it is determined that the forwarding hop count carried in the first D₂D synchronization information is less than the greatest value of the forwarding hop counts that is obtained after the update, the forwarding hop count in the first D₂D synchronization information is updated, to obtain the second D₂D synchronization information. The second D₂D synchronization information is sent, and the greatest value, obtained after the update, of the forwarding hop counts is also sent.

In actual application, because a signal on a transmitter device submerges a signal on a receiver, a device cannot send and receive a signal on a same time resource. To avoid that the device receives and sends a D₂D synchronization signal on a same time resource, the following three optional implementation manners are used.

Manner 1: When a D₂D synchronization signal is received by using any one of radio frames used for sending a D₂D synchronization signal, a D₂D synchronization signal is transmitted by using any one of the other radio frames used for sending a D₂D synchronization signal, where two or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured.

Manner 2: When a D₂D synchronization signal is received by using any resource that is used for a D₂D synchronization signal and that is included in a radio frame used for sending a D₂D synchronization signal, a D₂D synchronization signal is sent by using any one of the other resources that are used for a D₂D synchronization signal and that are included in the radio frame, where one or more radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, two or more resources used for a D₂D synchronization signal are configured.

Manner 3: After it is determined that a D₂D synchronization signal needs to be sent, only the D₂D synchronization signal is sent within a set quantity of transmission cycles, and no D₂D synchronization signal is received within the set quantity of transmission cycles.

Based on the foregoing technical solutions, in the embodiments, a device in D₂D communication receives D₂D synchronization information sent by another device, and synchronizes with a synchronization source corresponding to a synchronization source identifier carried in the D₂D synchronization information, to implement synchronization between the device and the another device; and when determining that a forwarding hop count carried in the D₂D synchronization information does not exceed a set threshold, updates the forwarding hop count in the D₂D synchronization information, and sends updated D₂D synchronization information, so that a device adjacent to the device can synchronize with the device, thereby implementing synchronization between devices in D₂D communication. In addition, complexity of communication between devices may be effectively controlled by limiting a forwarding hop count. D₂D synchronization information that has a same forwarding hop count and that is from a same synchronization source may be received after combination, so as to obtain a single frequency network (Single Frequency Network, SFN) receive gain.

A person skilled in the art should understand that the embodiments may be provided as a method, a system, or a computer program product. Therefore, the present invention may use a form of hardware only embodiments, software only embodiments, or embodiments with a combination of software and hardware. Moreover, the present invention may use a form of a computer program product that is implemented on one or more computer-usable storage media (including but not limited to a disk memory, an optical memory, and the like) that include computer-usable program code.

The present invention is described with reference to the flowcharts and/or block diagrams of the method, the device (system), and the computer program product according to the embodiments of the present invention. It should be understood that computer program instructions may be used to implement each process and/or each block in the flowcharts and/or the block diagrams and a combination of a process and/or a block in the flowcharts and/or the block diagrams. These computer program instructions may be provided for a general-purpose computer, a dedicated computer, an embedded processor, or a processor of any other programmable data processing device to generate a machine, so that the instructions executed by a computer or a processor of any other programmable data processing device generate an apparatus for implementing a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams.

These computer program instructions may also be stored in a computer readable memory that can instruct the computer or any other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory generate an artifact that includes an instruction apparatus. The instruction apparatus implements a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams.

These computer program instructions may also be loaded onto a computer or another programmable data processing device, so that a series of operations and steps are performed on the computer or the another programmable device, thereby generating computer-implemented processing. Therefore, the instructions executed on the computer or the another programmable device provide steps for implementing a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams.

Obviously, a person skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. The present invention is intended to cover these modifications and variations provided that they fall within the scope of protection defined by the following claims and their equivalent technologies. 

What is claimed is:
 1. A second device, comprising: a receiver, configured to receive first device-to-device (D₂D) synchronization information sent by a first device in a D₂D communication, wherein the first D₂D synchronization information comprises a synchronization source identifier and a forwarding hop count; a processor, configured to: synchronize with a synchronization source corresponding to the synchronization source identifier in the first D₂D synchronization information; and update the forwarding hop count in the first D₂D synchronization information in response to determining that the forwarding hop count in the first D₂D synchronization information is less than a set threshold, to obtain second D₂D synchronization information; and a transmitter, configured to send the second D₂D synchronization information.
 2. The second device according to claim 1, wherein: the first D₂D synchronization information received by the receiver is carried in a D₂D synchronization signal sent by the first device; or the first D₂D synchronization information received by the receiver comprises first-part D₂D synchronization information and second-part D₂D synchronization information, wherein the first-part D₂D synchronization information is carried in a D₂D synchronization signal sent by the first device, and the second-part D₂D synchronization information is sent by the first device through a physical D₂D synchronization channel (PD₂DSCH).
 3. The second device according to claim 2, wherein the receiver is configured to receive a D₂D synchronization signal using a first resource that is used for a D₂D synchronization signal and that is comprised in a radio frame used for sending a D₂D synchronization signal; and wherein the transmitter is configured to send a D₂D synchronization signal using a second resource that is used for a D₂D synchronization signal and that is comprised in the radio frame, wherein radio frames for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, a plurality of resources used for a D₂D synchronization signal are configured, and wherein the first resource is different than the second resource.
 4. The second device according to claim 2, wherein the transmitter is configured to, after determining that a D₂D synchronization signal needs to be sent, send only the D₂D synchronization signal within a set quantity of transmission cycles; and wherein the receiver does not receive any D₂D synchronization signal within the set quantity of transmission cycles.
 5. The second device according to claim 1, wherein the processor is further configured to: in response to determining that the receiver received a plurality of pieces of first D₂D synchronization information, separately acquire forwarding hop counts carried in the received plurality of pieces of first D₂D synchronization information; determine a minimum value of the acquired forwarding hop counts; and in response to determining that the minimum value is less than the set threshold, synchronize with a synchronization source corresponding to a synchronization source identifier in the piece of first D₂D synchronization information that corresponds to the minimum value.
 6. The second device according to claim ₅, wherein the processor is further configured to: in response to the minimum value corresponding to more than one piece of first D₂D synchronization information, determine signal strengths of each piece of first D₂D synchronization information that corresponds to the minimum value, and perform synchronization according to the piece of first D₂D synchronization information that corresponds to a greatest value of the determined signal strengths.
 7. A device, comprising: a processor, configured to: determine a synchronization source identifier and a forwarding hop count; and encapsulate the determined synchronization source identifier and forwarding hop count into first device-to-device (D₂D) synchronization information; and a transmitter, configured to send the first D₂D synchronization information.
 8. The device according to claim 7, wherein: the first D₂D synchronization information further comprises a synchronization source type; the device further comprises a receiver; the processor is further configured to: scan a downlink synchronization signal sent by a network; in response to determining that the processor has not detected the downlink synchronization signal, instruct the transmitter to send the first D₂D synchronization information; and in response to determining that the processor has detected the downlink synchronization signal, determine that the receiver has received third D₂D synchronization information sent by another device, determine, according to a synchronization source type in the third D₂D synchronization information, that a synchronization source is not a network device, and instruct the transmitter to send the first D₂D synchronization information.
 9. The device according to claim 8, wherein a time-frequency resource occupied by the first D₂D synchronization information that is sent by the transmitter is different from a time-frequency resource occupied by the downlink synchronization signal that is sent by the network and that is received by the receiver.
 10. The device according to claim 8, wherein the transmitter is configured to: send a D₂D synchronization signal, wherein the D₂D synchronization signal carries the first D₂D synchronization information; or send a D₂D synchronization signal, wherein the D₂D synchronization signal carries first-part D₂D synchronization information of the first D₂D synchronization information; and send second-part D₂D synchronization information of the first D₂D synchronization information through a physical D₂D synchronization channel (PD₂DSCH), wherein the first-part D₂D synchronization information and the second-part D₂D synchronization information are combined into the first D₂D synchronization information.
 11. The device according to claim to, wherein the D₂D synchronization signal comprises a primary D₂D synchronization signal (PD₂DSS) and a secondary D₂D synchronization signal (SD₂DSS).
 12. The device according to claim to, wherein the D₂D synchronization signal comprises a primary D₂D synchronization signal (PD₂DSS) or secondary D₂D synchronization signal (SD₂DSS).
 13. The device according to claim 12, wherein the processor is configured to: acquire, according to a preset correspondence between an SD₂DSS and a synchronization source identifier and between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined synchronization source identifier and forwarding hop count; and add the SD₂DSS to the D₂D synchronization signal, and use the D₂D synchronization signal as the first D₂D synchronization information.
 14. The device according to claim 12, wherein the processor is configured to: acquire, according to a preset correspondence between a synchronization source identifier and a PD₂DSS-SD₂DSS group, a PD₂DSS-SD₂DSS group corresponding to the determined synchronization source identifier, wherein each PD₂DSS-SD₂DSS group comprises multiple combinations of a PD₂DSS and an SD₂DSS; acquire, according to a preset correspondence between an SD₂DSS and a forwarding hop count, an SD₂DSS corresponding to the determined forwarding hop count; and determine a combination of a PD₂DSS and an SD₂DSS that comprises the acquired SD₂DSS and that is in the acquired PD₂DSS-SD₂DSS group; and add the determined combination of a PD₂DSS and an SD₂DSS to the D₂D synchronization signal, and use the D₂D synchronization signal as the first D₂D synchronization information.
 15. The device according to claim to, wherein the receiver is configured to: receive a D₂D synchronization signal comprised in one or more first radio frames used for sending a D₂D synchronization signal; and wherein the transmitter is configured to transmit a D₂D synchronization signal by using one or more second radio frames used for sending a D₂D synchronization signal, wherein a plurality of radio frames used for sending a D₂D synchronization signal are configured for each transmission cycle, and in each radio frame used for sending a D₂D synchronization signal, only one resource used for a D₂D synchronization signal is configured, and wherein the first radio frames are different than the second radio frames.
 16. The device according to claim 10, wherein the transmitter is configured to, after determining that a D₂D synchronization signal needs to be sent, send only the D₂D synchronization signal within a set quantity of transmission cycles; and wherein the receiver does not receive any D₂D synchronization signal within the set quantity of transmission cycles. 